I bought factories only to replace one when destructed, and I wait for a long time before upgrading technology.
So I cleared the whole map from trees and I bought nothing but snipers. When needed, I upgraded technology (but a few, just for fighting planes waves) and when the whole map is plenty of snipers, I just upgraded (feeling of becoming invincible ^^).
This make me go to the wave 500 and the game freezed. Without that I think I could survive 500 more waves, at the least =)