When I play HU SNGs, I play Pot Limit so that I don't have to worry about getting into an all in war with some of the idiots that sign up just to try and get lucky.
Anyway, I always start out aggressive, to gauge what type of player I'm up against. If they play back at me, I back off and try to make plays on them post flop. If they try to be the aggressor, I slow down and look to trap them. If they let me walk all over them, I start pounding.
One thing I have learned about heads up play is that you have to avoid calling. Preflop, you should be raising or folding.