Well, I've been stagnating around 60% win-rate in these things but decided several weeks ago that I could move up to the $20 level to test the waters. I actually have found it a lot easier than the $10 level for whatever reasons. I do think that part of it is that at the $10 level, your opponents force you into lots of coinflips on which it's hard to be much better than a 60% favorite.
I do feel like I'm getting a lot better at deciding when to proceed and when not to, when to bluff and when not to. Essentially, if my opponent is playing too tight, then I bluff at every opportunity and, if I get played back at, I lay down VERY easily (opponent probably has at least TP).
If too loose, then I just pick a spot and go. Also, if they're bluffing every check from the button, I check-raise a lot on middle pair or better.
I feel like those strategies, combined with Harrington's thoughts on raising have definitely improved my HU play a lot. Just a little bit more building, and I think I might give the $50 game a shot.