It's normal to get excited when you see a good buck (or any deer for that matter). But when a big buck comes along, you can't let that excitement turn into buck fever. When you know a deer is a shooter, don't spend time counting points, estimating spread or trying to decide how to have him mounted.
Look for a shot opportunity and take the first one you feel good about. You never know when a deer might smell you, hear you or change direction, so don't let a good opportunity get away.
Also, don't concentrate on how big the buck is. Just think about your routine and the things you need to do to make a good shot. Remember that you've done it time after time, and this is just one more shot. If you can take the buck out of the equation in your mind, you can beat buck fever.
Finally, if you feel yourself getting too excited, look away, think about the excitement and realize what's happening. Take a few deep breaths and try to relax. When you're ready, turn back and concentrate on the shot and not the deer.