If you can't beat it join it.
All the advice given has been great, but may result in not helping at all or at least not right away. Until you figure out how to control it so you can be succesful at shooting during buck fever you should try to manage it by practicing with it. Unfortunately there is NO way to duplicate the buck fever feeling you get.
The only thing you can do to come close is to get your heart pumping and your legs shaking and try to learn how to calm youself enough during those conditions to make an effective shot. Go run for .5 mile or so until your heart is pumping and your adrenaline is up. Then get in your practice stand and take ONE arrow and shoot at your 3d. Make sure you only shoot one arrow at it. You'll know you have one shot and mentally that will put you in a similar situation as buck fever. Physically you'll be in a similar situation. That's about as close as you can get to practicing under buck fever conditions.