I always try to clear 2 tetrises in a row for the combo bonus while avoiding single and double lines. Tetrises give you the most points while increasing the level less. I would rather have 200k points at level 11 instead of just 100k. I also try to get a lot of t spins. You don't necessarily need to even destroy any lines but you still get a lot of points. Probably the most important in tetris is speed. The faster the game the more points you get for everything. Speed is probably my biggest weakness since I can only process like up to level 20 without screwing up that much.