You need to avoid the main mistakes, for xemaple, players play every hand - it isn`t correct style of playing. At the first time you think the more you playing - the more you can win but it`s not true, usually it means losing more money.
Also you din`t need to bluff just for the bluffing`s sake. There is no any rules in poker that you need to bluff, so if you can`t do it in the professonal way, so you`ll loose and ruin your reputation as a competent player.
Always pay attention to the cards on the table and try to remember all cards.
Don`t play drunk, in a bad mood or disapointed, also do not play at too high limits nad always look at the level of championship you want to take part in. here is a lot of different levels of the professionality, so you need to pick up coresponding to your level of knowledgfe and experience.