These are gained from experience, and mainly re-iterate what you've probably already read, but have yet to learn the importance of.
DO use the player notes - an invaluable way of tracking the hundreds of faceless players you will meet, knowing their style of play when you first sit down at a table, may mean you can call their bluff, or save yourself a big bet, when you know they have you beaten.
DO learn about pot odds. It isn't easy but its essential. Read this detailed guide, then re-read, then learn it!
DO learn about the importance of position. Pre-flop hands can lose their strength when playing in early position (blinds), while being the dealer allows you to see what everyone else does first before you have to make your move, this is a powerful position.
DON'T go 'On Tilt' One of the easiest, and worst things you can do, is go on tilt, playing hands just because you haven't played any for ages, playing hands because you just lost half your stack when your AA just got beaten by an 83 offsuit, playing hands because you just cam ein from the pub and you've had a few too many to drink!
DON'T overplay a hand if you missed the flop, trying to play QQ when the flop comes A 10 3 is just not worth it if there is any serious betting going on. It's a bitch to fold top pairs, but you will lose a lot of money if you overplay a hand that you are just not going to win with (Trust Me!)