After struggling with cash games and full ring SNGs, I found that playing the heads up games is the only way I can consistently make money. I now have an ROI of about 9% over 100 games (not much I know but I've only been playing online for 2 months), but I'm only playing the $2 HU games.
I don't have much in the way of advice but I have found that at these low buy in tables people are too eager to push all in. I have gotten by through playing loose aggressive, but being really unwilling to go all in. I find this works out in my favour, a lot of the time people get frustrated with me and start pushing with middle pair or top 40% pre-flop hands, so I can wait for a good hand to call them with.
My problem is that I often have to give up on bluffs, I'll raise pre-flop, get called, bet 2/3 pot on the flop and get called or re-raised (with no pair), then I have to check the turn and usually get bet against. Perhaps I'm trying to bluff too much...
Did you find a significant increase in skill of your opponents when you moved from $2 to $5?