ditto on the beer. My biggest leak. I am a recreational drinker. I dont drink much during the week, unless a friend comes over (I always have a crate of good German beer cold). I am quite disciplined in my poker, my job, my life.
Having said that, I do love beer, and do drink regularly on the weekends. I see friends, go to a beergarden with my wife or friends, have a cold one in the evening when the kids are in bed. And the beer makes a new man out of me and that man wants to play poker. I think it is not an exaggeration to say that the major portion of my losses were when I was drinking. I do all the wrong things. I chase, I go up in stakes above my BR, I say "you fish, you got nothin" when I get aggessed on. Now, one or two, maybe 3, fine. But sooner or later the "new man" kicks in and he SUKS at poker.
I am getting better at leaving the PC off but it is still a leak, and I can only say that if you have a slight buzz, maybe it will not affect your game. Everyone is different, and some people dont lose their thinking skills, but gain confidence to make a play when it is the right time to do it - i.e. a passive player having the guts to aggress when it is actually the right time to - but too much will lose you money in the long term.