In sacrificing the exchange two things are important. The strength of the minor piece removed, and the strength of the minor piece that remains. I learned this the hard way in an online (turn based) game. Trying my usual 'mate down the h file' line against the Pirc, I got hit with a sac on c3 and was quickly in trouble. My opponents knights danced around the middle of the board, and after the timid Rhf1 I lost my material advantage, and the game soon after.
Press,Shaun - Nightflight [B07]
Chessworld online, 18.11.2011