Over the 10 years I have been playing I have noticed quite alot of common mistakes players make. I did some of them in the beginning too and learnt from my mistakes.
I thought to help all newbies I should start this thread and everybody can help fill it with mistakes to avoid. Here is my first tip:

Do not stay in a game just because you have put so much money in the pot. If you cards are not good enough you will probably just loose more. Give it up and get out in time.