When you are under pressure don't freeze and put a choke hold on the club. Stay loose! While waiting for your turn to hit do a few stretches and practice swings to stay loose and control the nerves. When setup over the ball, keep some motion in your legs and arms to combat the nerves - waggle the club and don't grip too tight -- then settle in to position and swing. In addition, make your swing thoughts simple and usually down to one positive thought like: Drive in the Fairway - Smooth Swing - Follow Through!
These tips can help control nerves and allow you to make a more tension free swing.