Practise, practise, practise.
If you can't get 200 lines on the marathon mode fairly easily then you're probably going to struggle with the more experienced players. Beating the computer on versus level 3 is a good target so give that plenty of practise, as very few player are any better than this.
Use the hard drop as much as possible.
Keep an eye on which piece is coming next and try to plan ahead so as not to leave any holes.
Don't clear a tetris as soon as it is available - build things up a bit first. Hold a long piece and, when another long piece comes along, use these to send two or three tetrises back to back in quick succession. If your opponent is doing the same (which most do) then they'll be in big trouble as each tetris sends four lines to their screen.
Don't ONLY take out four lines at a time as this can leave your opponent with a gap just waiting to have straight pieces dropped into. If you throw a couple of doubles and triples in before the tetrises it blocks the empty column and makes it harder to clear.
If you haven't used the hard drop you can rotate the piece across the pile and 'climb' over higher points in the pile. This is especially useful when you're almost beaten and can buy a little time if you're not sure where to put a piece.
I'm sure there's lots of stuff I've missed but that's all I can think of from the top of my head.
