What are some tips for shooting down spy planes?
They are fairly easy to spot [bigger than mw2], you just have to check out the skies for a moment. Here is a video, so that you know what to look for.
Location
They appear in any direction, but as far as I remember they fly clockwise with an approximately 60-70° angle from the ground.
Shooting them down
Keep an M72 or the Strela-3 ready as a secondary weapon to engage them without hassle, or focus fire with a light machine gun. If none available, they should go down in 2 cartridges of ammo from an assault rifle.