Here's how I do it.


Step 1: glue model on it's plastic base.
Step 2: glue model's base on a metal washer (mine are 2,5mm thick)
Step 3: put a little ball of greenstuff in the center hole of the washer...
step 4: put little drop of super glue on the greenstuff
Step 5: press a 2x5mm rare earth magnet in the ball of greenstuf
Step 6: Press down model on flat surface. the magnet sinks in the greenstuff and is level with the surface.
Step 7: put it away for a few hours to set and cure.
(for metal models I use 2 magnets)

Done!


Pro's:
-Plastic models carry a bit more weight and are nicer to handle
-Models stay upright and don't tip over due to the weight and low point of gravity.
-You can put them on a metal tray and they will never fall of, this is especially handy at tournaments because you just put them on a tray and don't have to worry about bumping into anyone and that you models fall on the floor. (see picture)


Con's:
-time investment during assembly
 -Models are 2,5mm taller

Pictures:
In the last picture you can see I hold the tray at a near 90 degree angle. (tray with metal sheet but a metal tray works even better) I can flip the tray over 180 degrees and the models still stick. Even metal figures like the heavy British para medic at the left front. (2models on 1 base)