If you play low-stakes no-limit online, you're probably playing in a six-max game. When I used to play on Absolute Poker, before the absurd scandals there (read all about it
here, or follow Nat Arem's investigation (
part 1) (
part 2) (
part 3), or
listen to reportage here), six-max games were almost all that was available for no-limit. Now that I'm playing at Full Tilt, there's perhaps a fifty-fifty balance between 9-handed and 6-max.
I always struggled in 6-max games, at both limit and NL, because I had learned to play "the right way" from authors like T.J. Cloutier, Dan Harrington and David Sklansky. The right way, of course, meant waiting for cards and then value betting them. The problem is, at least one-third of your time in six-max games is spent in the blinds, and you simply don't get a hand frequently enough to make up for that disadvantage. When you
do get a hand, it's more likely that you'll be out of position, since you're more likely to be UTG or in the blinds - these positions are half of the six-max game, compared to one-third of the available seats in a full ring.
I was frustrated, because I knew my opponents were playing "too loose" but I still wasn't winning. However, I've been able to gradually turn that around. The book that's been most helpful has been "Killer Poker Shorthanded" by
John Vorhaus and Tony Guerrara:

I'd been pretty skeptical of the "Killer Poker" series. These books generally constitute a whole shelf in the poker section of the chain bookstore of your choice, and I was afraid they were the pulp fiction of the poker world: cranked out quickly, amusing, but insubstantial.
I was wrong, mostly. They are amusing, and they may be cranked out quickly, but there is some good information here. Not at the level of detail of the Ed Miller/David Sklansky/Mason Malmuth applied mathematics books from two-plus-two, but very useful nonetheless. The authors focus more on the mindset and adjustments necessary for six-max play, and advocate a "play-the-player" approach.
Killer Poker Shorthanded is new and I'd recommend it. While it may be unhelpful for better players in $100NL and up, for people like me at $10 NL it's invaluable.
I'm not the only one who thinks this is a good read, check out
Tim Peters' review in Card Player here.