I've been playing cash games, tournaments, and online for a couple of years. After playing countless numbers of hands, I've complied a list of important tips that all players should take heed to:
1. Never play with money that you can not afford to lose - Before you sit down this should be the first thing on your mind. If you can not afford to lose, do not play. You will not be sufficiently focused, or aggressive enough to play the game that you need to play in order to win. Why? Because you will be too scared to lose that money.
2. Do not play too many hands - Hand selection is the single most important factor preflop. Try to make those premium starting hands win for you. The better cards you play preflop the more confident you will be. Ever notice the guy that plays everything? For about three hands he is the chip leader, one hand later he is on his way home. Do not be that player.
3. Do not just call - In very few situations is it correct to just call. If you are not betting, or raising, most likely you should be folding.
4. Stop trying to bluff - Against a pool of solid players, bluffing is an important skill that must be utilized. However, at low limit stakes and most tournaments there is someone waiting to catch a blink. I can not count the number of times I've broken and seen a guy broke because he calls everything off with second pair. With that type of action, bluffs simply do not work. (Remember low limit stakes, freerolls, etc.)
5. Do not insult the fish - Remember the dialogue from Rounders, "IF you sit down at a table and within a half you do not know who the idiot is, then your that guy." Do not be the one calling a guy a Donk in the chatbox, or laughing, calling someone a fish. For all intensive purposes these fellows pay your bills.
6. Avoid large pots (unless you have the nuts) - It is easy to get into a large pot. However, I'm not going to be the guy that loses his entire bankroll with top pair. AA Is the nuts preflop so you want to get your money into the pot. After the flop is where a player will make his money. Try to avoid big pots unless you have the nuts at each stage of the game. For example, You're dealt Kd Qd and the flop comes Ad 5d 8d. Right now you have the nuts. Absolutely you bet, you could even get a reraise this is a perfect scenario. However, if the turn pairs the board you might be in trouble. There are a million different scenarios but my main point is to try to get all your money in when you have the absolute nuts.
7. Tilt is your worst enemy - You are going to have suckouts, bad beats, lucky loses, and everything in between. That's poker. The way that you react is what will separate you from the fish or you from your bankroll. Tilt is the easiest way to lose everything. You usually feel it after a suckout or lost big pot. My advice is to take a break. Trying to get back what you just lost is a recipe for destruction. Everyone else that you're playing with realizes it too. They are out for blood and you're the main dish. Do not tilt your money or chips away!
8. Play your position - This is easily said but hard to do. Those premium starting hands look pretty, but in the early positions play cautiously. It is easier for a guy to call a raise after someone else has called because of pot odds. Play strong starting hands aggressively in later positions but beware in the earlier positions. Simply because there are a lot of people behind you, and after the flop you will be first to act that puts you at a disadvantage.
9. Pay attention - Even when you're not involved in a hand pay attention to what's happening. This will give you valuable information on players. Pay attention to how your competition plays, his mannerisms, the tone in his voice. All of these factors will help you develop a strategy on how to beat your opponents.
10. Be a gentleman - Being a gentleman is worth more than it's weight in gold. No one likes a loudmouth, or an obnoxious individual. When you're trying to win you do not want to be a target. Writing insults in the chat box is childish and ridiculous. If you're playing live, tip the dealers. All in All try to be a respectable winner. Do not insult the person that you just beat. Play your game in a gentleman like fashion and you will see the rewards almost immediately.
This list is not the tell all manual to winning in NL Holdem. However, it is a start to getting you on your way. By taking these simple tips into mind you will have an enjoyable poker experience.