Drinks
When I was a young harp player I would get so nervous when sitting in that I would get dry mouth big time for the first couple of songs. I searched for the perfect drink that would help keep my mouth moist... something that would help me salivate, but not too much (we all know how bad too much spit is). When I opened for Cotton (I was 18) for the first time believe it or not I had a slice of lemon sitting on my amp... I wasn't going to get dry mouth opening for Cotton! ;-)
Over time I found that nothing really worked well. I found that just drinking water (but not too much out of nervousness before I would play) worked just fine. Worse case take a full water bottle and add one drop of lemon juice.
What helped the most was to control my breathing and say to myself... relax (you always play better when relaxed)... play musically (don't force anything... don't play to impress)... play repetitively (the key to playing memorable music). I still say this to myself before I play each show.