Affordability: If you can not afford to lose the money that you have, then do not play poker with that money. While it is true that poker is a game of skill, the skill factor only manifests itself in the long run. In the short term, luck matters more than skill and therefore you need to make sure that a run of bad luck will not ruin your life and the only way to do that is to make sure that you never play with money that you can not afford to lose.
Folding Ability: Unless you somehow have become the best bluffer in the world, the word that you are going to want to announce most often pre-flop and post-flop is fold. Your ability to win or to break even when you are a beginning player is directly related to your ability to fold bad hands and only play good ones. This means that you only play the most solid starting hands possible and it also means that you do not continue to chase unless you have the pot odds to justify calling.
Proper Calling: Have you heard the terms fish and calling station before? These are terms that are used to define players that for some reason can not seem to part with a hand that they have been dealt. If you are one of these players, expect to lose your stack pretty quickly. If you can't seem to stop calling with marginal hands, then play raise/fold poker; always raise or fold, but never call.
Cultivate Maniacs: Maniacs are players that always need action and therefore are always calling and raising. These people are your friends, because they are going to play with abandon whenever they get the chance. Wait for good hands, trap and make money off maniacs whenever you can.
Bluff less, semi-bluff more: When you are playing online, especially at lower limits, you will find that bluffs are not going to work most of the time. Likewise, if people are betting on the river, generally they are not going to be bluffing. This means that you want to bluff less, but semi-bluff more.
Money management is the key: Play with only a small amount of your total bankroll at once and be careful about considerations for moving up the limits.
Check sparingly: Free cards can come back to haunt you if your opponents have straight or flush draws.
Don't Drink: It will mess with your intuition and logic.
Deal with tilt fast: Players on tilt are capable of some horrendous self-injury, including blowing through their whole bankroll in a very quick timeframe. The sooner you deal with tilt (even removing yourself from the game if necessary), the better of your prospects will be when you recover from it.