Not making enough value bets (Winning the small pots), and calling all-ins (losing the big ones). Everytime I see a fast all-in, there's always that little thought "he's trying to buy-it, that stupid bastard!". I think the next step is learning to think "HE JUST PUSHED ALL IN---HE HAS A HAND". And When I do have the hand, I tend to CB the Flop, and no one calls me. Ever.
