I play a lot of Heads Up Tourneys online at Full Tilt. As you all know, some people like to push all in quite frequently, especially if you have raised to double, triple, or 4x the big blind pre-flop. When trying to decide whether or not to get all my chips in the middle, I consider a couple main factors: the skill level of the person I am playing against and the blind level that we are currently on. I would say that I am the dominate player in the match 95% of the time, therefore, I would rather avoid potential coin flip situations and instead use superior play to grind down my opponents. Because of this, in early to middle rounds, I rarely call an all in push with less than Qs, Ks, or Aces, even if I feel that the other player is pushing too often and I might be a slight favorite. It just insn't worth the risk if your skill set is superior. As the levels advance, if I haven't been able to vanquish them, I open up my requirements as time goes on.
I made a more detailed post on this subject on my blog at
www.lasvegaspokerchick.blogspot.com I condensed the post here, but I would love to hear any comments on this summary or the entire post.