how to beat the trompowsky?

Why are people bumping 16 year old posts? Yikes.

If you're trying to play for a win against the move 1 d4 Nf6 2 Bg5 c5 3 dxc5?!, the best way to do this is not 3...Na6, which is only equal, but 3...e6, which gives White more chances to blunder. Both moves lead to equality with BEST play by White, but 3...e6 can lead to a miniature if White tries to hold onto the pawn with 4 b4? a5! 5 c3?! (5 b5, accepting a small disadvantage, is better). axb4 6 cxb4 Nc6! 7 Qd2 b6!, and White is in serious danger of losing in a few moves. (8 cxb6? Bxb4 9 Nc3 Ra3 10 Rc1 h6!). And the critical line, which requires you to have forseen the correct counter move, is 8 b5 h6 9 Bh4 (obviously 9 Bxf6 is suicidial) Bxc5! 10 bxc6 (black has sacrificed the knight) Ra4!!-the only move which justifies this sacrifice. Then 11 Bg3 loses to either 11... Rd4! (which will be followed by ...Bb4+ and Ne4 soon after) or 11...Ne4 (which may be even stronger!) 12 Qc2 Bb4+ 13 Kd1 dxc6! 14 Kc1 (14 Qd3 0-0 and White is toast even if he trades queens) Ra5! 15 Qxe4 (White is now up two pieces!) Rc5+ 16 Kb2 (16 Qc2 Qd4! is hopeless). Qf6 and White is getting checkmated after he loses all of his extra material.