Be More Selective: While there are players around such as Gus Hansen and Sammy Farha that are able to get away with playing lots of hands, what you need to realize is that these players are some of the best in the world at being aggressive. For most people, aggressiveness is not their natural style and therefore it takes time and practice in order to develop it. Until you get to the point where you can be loose and aggressive in your play, you need to be very selective at picking your spots. If you are playing at a poker table with ten other people and are playing more than three hands each round of the table, then you need to be more selective.
Learn Pot Odds: Learning the theory of pot odds and how they relate to the hands you are drawing to is essential to playing poker at a professional level, because pot odds is a lot like trading stocks. If the return is greater than your chances of getting to the hand that you want, then you should call because the play is net positive over time.
Practice Reading Skills: If you are truly interested in improving your game, then you need to become a better reader. While reading online is harder sometimes than reading in person, at the same time online tells are there. This is most obvious in the case of betting patterns and sometimes just observing a table and seeing if you can guess a person's hand based on how they play can be a useful exercise.
Raise Real Estate: If you are in late position, try raising the blinds a few times to see how those players play.
Avoid smaller pairs: This is especially true in limit, where implied odds are not as great as they are in no limit. Sometimes, folding smaller pairs is a very good idea.
Learn implied odds: If you are not quite there with pot odds on a specific card, you can factor in the amount you think you can win on the next card if you hit your hand; this is known as implied odds.
Read Hand Histories: Becoming a better poker player means learning from your mistakes; hand histories help you accomplish this.
Experiment with different styles: This helps you see how different playing styles suit your game and it also helps you change gears when you think someone at your table might have you pegged.
Semi-bluff the turn more often: Raising on the turn with position against one opponent and then folding to a re-raise or a bet on the river costs you 2 bets; the same as if you were to call on the turn and river.
Have fun: If you're not having fun, then chances are you are on tilt and that is not good for your game.