I used to drink all the time at a show, before, during, and after until the wee hours of the morning. Then I began scaling that back so that I was not drinking during our set. I would have a beer or two before stage depending on how long we had before taking the stage. A rule of thumb that I followed was one beer per hour and no beer inside the hour preceding our set. Now I have gotten to the point of where I do not consume any beer prior to stage and only have a few after we have all of our gear off stage and safely stowed.
We record our shows and I compare performances afterwards to see where I can make improvements in my playing and performance. Sometimes it is really difficult to watch myself as I am my own worst critic. One common thread I have found when it comes to poor personal performance is consumption of alcohol prior to playing. This is not a judgement of others - merely a personal observation. I am just not one of those musicians that can pull off drinking before playing and do well. I made a personal choice to refrain from drinking before playing for the sake of presenting my best at each show.