Another trick is if the ammo bunker in the town is burning, the top of the smoke is the minimum altitude to drop. Without the smoke, you have to do the math and make sure that the troops are let out at a minimum of 800 Above Ground Level (AGL).
From an internal view, if I see the bunker smoking, I fly to the top of the smoke and as soon as I hit that, I pull straight up and release troops at the same time. Pulling straight up produces the "hammerhead" drop like dfl8rms described. Works every time.