Things not to drink before singing. Milk, beer, or any alchohol. Things that are ok: Water with lemon, Tea with lemon. Avoid caffine if possible. Caffine will cause your throat and jaw muscles to constrict or cramp up. Don't use cough drops, cloriseptic, etc...they will damage your throat over time. Obviously avoid smoking.
Conciously keep yourself as relaxed as you can. It helps me hit the higher notes with out a lot of strain. Try to warm your voice up as well prior to performing. All that said, I am not a vocal coach. I did however log 14 years of over the road stage time as a player/vocalist. A vocal coach may be just the thing for you. They may be able to help you.

Kris