Probably the number one mistake beginning poker players make is that they play far too many hands. When you're just starting out playing poker, you want to play poker, and that means staying in hands that aren't very good just to be part of the action. But playing more doesn't mean winning more, it usually means losing more. If you find you're staying in half or more the hands you're dealt, you need to upgrade your starting hand requirements.
2. Don't Play Drunk
Countless nights have I sat across a table from someone & watched them get plastered silly and throw away their entire stack of chips. I've been that person too - and there are nights where you're just playing with friends for low stakes and it's more about the fun than the poker - but if you're in a casino, watch the alcohol. The truth is, while you may be more relaxed after 2 drinks, it may lead to you playing looser and less sharply, even if one's not 'drunk.'
3. Don't Bluff Just For Bluffing's Sake
A lot of beginner's understand that bluffing is a part of poker, but not exactly how. There's is NO rule that one must bluff a certain amount or at all during a poker game, but many players don't feel like they've won unless they've tried a poker bluff. Bluffs only work in certain situations & against certain people, and if you know a player always calls to the showdown, it is literally impossible to bluff that player. It's better never to bluff than to bluff "just to bluff."
4. Don't Stay in a Hand Just Because You're Already In It
Another common mistake beginners make is to think that "Well, I've already put that much in the pot, I have to stay in now." Nope. You can't win a pot just by throwing money at it. There may be cases when pot odds warrant a call, but if you're sure you're beaten, and there's no way your hand can improve to be the best hand, you should fold right away. The money you've already put in the pot isn't yours anymore, and you can't get it back just by playing a hand all the way to the end.
5. Don't Call at the End of a Hand to "Keep Someone Honest"
This one follows the last tip. I see a lot of players look at another player's final bet, look at the hand, & say "I know you've got me, but I have to keep you honest," as they throw in a final call. It may be worth it to see if a player really has the hand if you're not sure & you're gaining information that will help you later on, but if you really feel a player has the hand he's representing & you're beat, why give him another pile of your money? Those bets will add up over an evening.
6. Don't Play When Mad, Sad, or in a Generally Bad Mood
When you play poker, you shouldn't do it to escape from being depressed or having a really bad day. You start out on tilt -- playing emotionally, not rationally -- and you won't play your best. Likewise, if during a poker game, you lose a big hand or get sucked out on and feel yourself going on tilt, stand up & take a break until you feel calm later on. Fellow players will sense your mood & take advantage of it.