The most common mistake people make is to play too many hands. This just means you are playing bad hands and throwing money away. Use discretion and try to only play one out of every four or five hands you're dealt.
You also have to be ok with the concept of folding. Holding onto a hand just to see what happens or because you already put a small bet in is never a good idea and will result in you losing your money and losing it quicker.
You should set realistic expectations and should sit down at tables that match your skill level and bankroll. A table with a higher limit not only may be too risky for you, but it will probably have players who are far more advanced than you as well.
If you're just starting out in playing, trying to play with people who do it for a living or who are very experienced is just asking for trouble.
Another tip is that you can learn about your opponents when you bet against them. Sometimes even if you can't win a hand, studying your opponent's moves is valuable.
This is because people have tendencies that they follow and play in a specific way. You can protect yourself by making defensive bets so you won't face a big raise later. Meanwhile, re-raising your opponent can help weed out bluffers from those with good hands.
You also have to master the art of bluffing. If you bluff too much however people won't take you seriously so only use the tactic sparingly to ensure its success.
Bluffing should be used when you have the least risk of suffering from it. Additionally, your position on the table can have a strong influence on the way you should be betting and the way others will play against you.
There are tons of books and guides out there that can help you learn how to play poker better. Read many different ways to win and figure out the ones that work the best when you use them. You can keep track by keeping notes of your hands or using software that automatically does it for you.
When you know how to play poker, not only will you have more fun, but you'll also be able to win more money.