1) Take control preflop – Try coming in for a raise whenever you play a hand. I know this is tough, but taking initiative every time you open a pot will be very important to becoming a successful player, especially in no limit hold 'em.
2) Controlling tilt – Try to not care. As hard as that sounds you have to want to win but be OK with losing. I have losing weeks sometimes, and I try to remain positive. It always comes back around but if you tilt and continue to play it just will not work out.
3) Exercise! – Most people realize you have to be mentally tough to succeed in poker, but people do not realize the physical aspect of it. Most players do not take the time to work out or eat well, but let me tell you, it helps. Last year I made a sick run online and it all happened after I started working out. I also have gotten a new plan for eating well and working out, and since then I have been playing well and winning.
4) Do not fall in love with your hand preflop – If you were playing a tournament, and you raise with TT and I re-raise you back. Chances are your TT is not very good (in the first few levels of a tournament). Try just folding this hand, because you are essentially set mining and have no idea where you are in the hand.
5) Taking breaks – I have written some articles on this, and find this to be supremely important while playing poker . Last year during WSOP I played 6 events, cased in zero. I had three bubbles and the last tournament I made a dumb play to bust. The next day was an event I really wanted to play, the 2500 PLO / PLH mixed event. I decided to just drive out to Cali, and go to the beach and relax. So I did, I missed the tournament I really wanted to play so I could get my mind right. Low and behold it worked, the first event I played after I got back I took 3rd in a 3k buyin '.