Really know your stuff and present only what you believe. Rehearse alone and then try it out with family or friends. If you are worried about blanking out, use a flip-chart or index note cards to outline your topics so you don't have to remember what's next. Slow, deep breathing is very calming and will reduce your heart rate and relax the rest of your body. For a few hours before your presentation, avoid stimulants such as cigarettes and coffee.
Stimulants worsen anxiety.
If your topic allows for it, have fun. Admit that you're nervous. You can break the tension with good old humorous honesty. Before your presentation, talk out whatever negative thing you fear. Most fears are not rational.