I don't preshade. I don't find it controllable enough. Way too easy to under or over-cover with the top colors.
Instead, for armor, I paint the whole thing black, black-green (i.e. Tamiya NATO Black), dark brown, etc, then go in with the main color and just go lazy around the edges, leaving hints of shadow and depth.
Depending on the vehicle, I may lighten the main color and spray that more center-panel. Didn't feel like it on the T-34, though!
For aircraft, I use a technique I call the three-layer blend. Same effect as pre-shading, far more controllable.
Step 1: Paint your base color:
Step 2: Lighten base color with white, light gray, light tan, whatever blows your skirt up. I usually lighten by as much as 50%. Spray this in the center of panels, staying away from panel lines. It's a lot easier to AVOID panel lines than it is to follow them perfectly. You can also NOT lighten some panels, or lighten with different colors to add randomness, etc. Either way, it'll look nasty at this phase:
Step 3: Go back to your original base color. Thin the heck out of it. 75% thinner to 25% paint. Spray this on in light coats until the base and lightened areas start to blend.
Here's halfway through:
You can stop here if you want the contrast to be stark. I kept blending:
Final results under clear coats, wash, weathering, etc:
On the Bench: 1/32 Trumpeter P-47 | 1/32 Hasegawa Bf 109G | 1/144 Eduard MiG-21MF x2
On Deck: 1/350 HMS Dreadnought
Blog/Completed Builds: doogsmodels.com