Deployment
Keeping Tau out of combat starts early. Deploy poorly and you could be losing units a lot earlier than you should. Pre-measurement is your friend here. Remember that most of your weaponry has decent range so there's no need to deploy your forces very far forward at all. Most combat orientated armies are going to need to close range on you quickly so you needn't worry about being out of range as it won't stay that way for long.
Whilst you'll want to keep your units close together for Supporting Fire to be effective you need to make sure that you limit the ability of your opponent to multi-charge. Giving that expensive combat deathstar a single unit a turn is fine but you don't want to lose two or more to it in one round of combat. It's also worth thinking about your key units and making sure they're not deployed too close together. For example if you've got two pathfinder units they're probably best on seperate flanks. They'll have enough range to hit most things and your opponent must split his forces to deal with both.
Try to set up so that each unit has a couple of other units giving it Supporting Fire. This isn't always easy but can really pay off when the time comes.
Keep Moving
The changes of 6th edition were a real bonus for Tau. Firstly, rapid fire weapons can now move and shoot at full range and pulse rifles get their 15" two-shot range back. Secondly, pre-measuring allows you to ensure you're pretty much spot on 15" away. This means that most units on foot will move 6" and then need a 9" charge to get into combat. If you're in some terrain then this is a pretty tall order and you should be safe. Of course you'll be moving through terrain to keep away too though so unless you can gun them down you're still likely to end up in combat on the next turn.
Crisis suits have a huge advantage over their fire warrior comrades. The ability to jump-shoot-jump (JSJ). This allows you to keep your enemy at arms length for a long period of time. Don't forget that deep striking suits can now make an assault phase thrust move even on the turn that they arrive. Once again, pre-measuring is your friend here. By keeping at 12" range (for your plasma) and then jumping back to just over 18" (assuming you rolled well) you can make it nearly impossible for foot slogging units to ever make it into combat with you. I'll talk more about crisis suits in detail in a separate post soon.Riptides have the ability to Nova for a 4D6" thrust move. Considering you're unlikely to use Nova for your ion accelerator, this is probably the best use. Whilst your Riptide is hard to kill in combat he's likely to end up stuck there as you're unlikely to be able to rescue him. Therefore keeping him out of it all together is probably a good idea. Should you find yourself in a situation where combat is unavoidable, you're probably best using Nova for a 3++ save to try and hold up a combat unit for as long as possible.
Sacrificial Units
I have to say I'm not a fan of sacrificial units. I don't like to put anything into my list that doesn't perform an offensive role. Sometimes it's necessary but most of the time it's points wasted in my opinion. That being said, in the time of the old book a lot of Tau players used Kroot as "bubble wrap".
The idea with the Kroot screen is that you set them up in a line in front of your main gun line and create a speed bump to slow down the enemy attack. The aim being to buy some time to get your other units back and keep them out of combat. Kroot die nice and easily so your opponent should be back in the open ready for your next shooting phase. It's by no means a terrible tactic but I have a couple of issues with it.
Firstly, I think in 6th edition it doesn't really slow your opponents down much. The enemy units will be able to charge forward into combat, pile-in and consolidate after easily killing the Kroot. This will give them an extra move that wouldn't have been possible without the Kroot screen. Most enemy units will be 6" and then either shooting or running an average of 3-4". This means you'll still get another round of shooting at them anyway without needing to sacrifice the Kroot.
The other issue is that fast moving units with jump packs e.g. BA assault marines can probably hop over your screen and ignore it completely. If you keep your Kroot close to your gunline to prevent this they're not doing much to keep your enemy at a distance anyway.
Distraction
The Tau codex isn't short of units that can be used as a distraction though. For me this is a much better method of keeping an enemy army away from your gunline. Two units spring to mind for this, Kroot and Crisis suits.
By outflanking Kroot towards an objective that is held by a flimsy enemy unit you've got a good chance of taking it for yourself. Your opponent will struggle to ignore this and all the while your Kroot can be shooting at anything they can hit and hurt. This means your opponent is likely to pull back some of his forces to deal with them. That's less stuff coming at you.
Crisis suits can do a similar thing by deep strike. Whilst they can't claim objectives they can be pretty devastating to units controlling them. Either that or destroy tanks with ease. Your opponent will have to deal with them and they'll require more of a commitment than Kroot. Not only that but they're pretty mobile so if your opponent doesn't take the bait they can still get into the fight pretty sharpish.
The problem with these tactics is that they require units to be held in reserve. It's always worth bearing in mind how much of your army will actually be deployed. You want as much firepower on the table as possible at the start of the game. Whilst reserves have an 88% chance of coming on by turn 3 there's still the possibility that you won't get them until turn 4. This is great for Kroot objective grabbing but not so good for crisis teams.
Overwatch and Supporting Fire
For most armies overwatch is a nice bonus but isn't particularly effective at preventing combat. The reasons for this are two-fold. Firstly, most standard infantry shooting is S3/S4 which, after BS1 and then saves, doesn't result in many deaths. Secondly, there's no way to boost BS1 and the unit you want to shoot with aren't always the one being charged.
The Tau book allows for both of these issues to be addressed. Having standard infantry with S5 shots is pretty unique in 40K. This alone makes Tau overwatch fire better than any other army. Still a unit of 12 fire warriors is only likely to kill a single marine. However, assuming there's an ethereal around that 12-man unit will be firing 36 shots. That unit alone isn't devastating but if you've got Supporting Fire from another couple of units then you can be killing 3-4 marines. This is often more than enough to prevent a charge.
Finally, if you've got some pathfinders around they can overwatch with their markerlights. A unit of 10 is only going to score a couple of hits but even a single marker hit can mean another dead marine from overwatch.
Of course this is all based on shooting MEQ targets. If we're talking about orks, dark eldar or tyranids then there's the potential to completely wipe out a unit with shooting in your opponent's turn.
Surviving Combat (or not)
So we've done everything we can to avoid combat but in the end it's pretty inevitable that some of your units will get charged. Conventional wisdom is that you want to lose the combat and get wiped out to allow the rest of your army to keep shooting. However, in the new codex I think there's a case for trying to stay locked for a couple of rounds. The key thing is the number of rounds you end up locked in combat. Odd numbers mean you'll get a turn of shooting at the enemy unit. Even numbers mean your opponent stays safe for a turn and is free to charge something else.
The most likely units to get charged are your Fire Warriors and Pathfinders. Both of these units have defensive grenades. Don't forget they have a dual role, firstly in giving your squad stealth from the assaulting units shooting (within 8") and secondly reducing the number of incoming attacks. This creates the potential to survive rounds of combat and keep assaulting units locked and unable to kill more of your units. Now this is pretty situational but not impossible. It's certainly worth thinking about.
That's not to say you have much control over whether you win or lose a combat but in the past people took small units of fire warriors to ensure they lost combat. These days I reckon big squads are better for more overwatch and more chance of surviving a couple of rounds.
Gadgets
Finally, there are a few gadgets worth mentioning that can help with assaults. For crisis suits there's the Repulsor Impact Field, Counter-fire Defence System and Vectored Retro-Thrusters. The counter-fire system is probably a waste of a slot which could be used for a more useful support system.
The repulsor field is a one per army option but 10pts for D6 S4 I10 hits isn't too bad. It won't stop someone charging but can help limit the attacks coming your way. Bear in mind it can be used once per attacking unit too so multiple attackers can be hurt. Combine this with vectored retro-thrusters and you've got a good chance of surviving a charge and leaving combat to get more shots in. Throwing in a drone will give you I4 to improve your chances of getting away.
Darkstrider allows a unit to consolidate D6" after firing overwatch. Assuming you killed a couple of models with your overwatch this could easily be enough to prevent a charge happening altogether. Elsewhere pathfinders can take a grav-inhibitor drone to reduce enemy charge ranges by D3" which again might be enough to prevent a charge.
Finally, vehicles have a couple of options to help prevent assaults. Firstly the Flechette Discharger causes S4 I10 hits on models in base contact with the vehicle, again not going to prevent a charge but might keep the vehicle alive. The Point Defence Targeting Relay allows Overwatch and Supporting Fire from a vehicle's S5 or less weaponry. Both of these upgrades are probably a waste of points but worth noting all the same. Don't forget that Longstrike gives his vehicle Overwatch/Supporting Fire and it can fire several times per assault phase and includes the main gun! Whilst we're talking about vehicles it's still worth keeping them moving to reduce the hits coming in.Conclusion
There you go then. As a Tau player there's plenty of tools at your disposal for preventing combat. The key is deployment and pre-measuring. Never allow an assault orientated army to multi-charge if you can help it. Ultimately the best defence is to focus your fire on the biggest threats to eliminate them before they even get close!
All of this depends on your play style though. Most of what I've said here assumes you're playing a gun-line Tau cadre. The mechanics are very different for a mechanised force. Still I hope this article has been of some use to people.