Aside from having or acquiring poker knowledge there are several things unrelated to poker that can be done to help you play your best.
Here's a list of things I do in no particular order:
Eat right. Ideally I eat five small meals a day (it doesn't always work out this way). I try to avoid sugar and soft drinks.
Stay hydrated. If I'm playing a session, I drink about a glass of water every hour I'm playing. It helps me stay alert and concentrate.
Exercise. I try to get some form of exercise every day whether it be walking outdoors or using the eliptical machine. I believe 30 minutes is ideal and that's usually what I do.
Sleep. I try to sleep at least 8 hours every night and lately it has been about 9.
Eliminate distractions. This used to be a problem for me. I would be watching tv., making sports bets, talking to people, and even playing video games when I was playing. I do none of this anymore. I can't even begin to imagine how much those things cost me.
Listen to music. This always seems to help my frame of mind when playing and also gets me through those 4 and 5 hour sessions where I'd otherwise get bored. It also helps me concentrate and stay focused. I either listen to playlists in ITunes, AOL Radio, or Pandora.
Shut off my chat. I've always found the chat box to be detrimental and a distraction while playing. Yes, there's the occasional piece of information like ****** *****, that might lead you to believe your opponent is tilting but I'd much prefer to concentrate on how that person is playing before I draw conclusions about their emotional state. Besides, the vast majority of the chat on poker sites is, "You lucky mf fish, you *** ****", "You're the worst player I've ever seen", "You suck", etc. etc. It's an unfortunate reality that I'd rather not be a part of.
Take breaks. Any time I don't feel I'm playing my best I quit. This can be from a lack of concentration, fatigue, not thinking clearly, being upset about losing, etc. This has been one of the difficult things about having to play ~2200 hands a day. There are some days where I just don't feel right and I end up forcing myself to play because I need to put the hands in. I try to avoid doing this if it all possible though. I usually break up my sessions and take at least two breaks throughout the day. If I'm having a really bad day, I might take 3 or 4 breaks.
That's all I can think of right now.