Practice Fighting with your friends, use pads. Study various fighting arts such as Jujitsu, atemi waza, etc., as well as some of the more modern teachers of street fighting tactics and techniques, and incorporate from whatever you study only that which is useful to you. For example, if you're not very strong on kicking techniques or joint locking techniques then do not incorporate them. You want simple, easy to use tactics and techniques that are applicable to a wide variety of different situations that do not take years of practice to master. When you are confident that you are capable of utilizing what you have learned and practiced, instead of letting the thought of what an assailant might do to you cause you to freeze up, rather, concentrate and focus on what you are going to do to him/her. Also, don't give your game away, make the assailant think that you're afraid, help build his ego so that he may relax and slightly let down his guard, then, when he least expects it explode into action. But the best advice is do not put yourself purposely into situations or environments where you might be vulnerable to attack or ambush in the first place.
Addition: Here is a link that will tell you the effects of high stress self-defense situations on performance, and how to help counteract them.