If you have trouble exercising enough to get relaxed in time, whether there are physical or time constraints, there are two other techniques that may compensate.
Massage may be the oldest form of medical care and is shown in 5,000-year-old Egyptian tomb paintings. Getting a massage on a regular basis from a licensed professional is a good way to force yourself to relax, but even getting a shoulder massage from a friend right before you go on stage can help.
If nothing else dissipates the tension, consider exercising your lungs. Go where no one is likely to hear you — like a stairwell or by the trash in the back of a building — and just let out a full-throated scream as long as you can. You can do it more than once, if you feel the need, and you should feel better afterward (as long as no one calls the police!).
Once you have used various types of exercise to put your body in a more relaxed state most of the time and know what to do right before you speak, you can turn to psychological techniques that will also help.