I have trained in various martial arts all my life and have found that nearly all instructors will tell you to keep your calm in a fight to win, however I have found that there is no such thing as keeping calm in a street fight/mugging.
After all this isn't no competition and there are no rules you are instead fighting for your life and that is why it's next to impossible to keep your calm in a mugging.
For this reason nerves will play a major part in any street mugging, however you don't want to let them get the better of you and this is where you can use your anger to help you.
If you perceive that you're in any danger from an attacker approaching you from behind then turn around and face them and look at them right in the eye.
If they're a threat chances are they will look to attack you at this point before they do however scream and shout at them, this will be enough to put some people off but if they carry on don't worry you would have already changed their game plan slightly.
Now that you have shouted at them they may not be sure if they want to attack or not, so you have put a doubt in their mind, but because they are scummy people then they will in some instances keep on coming forward.
At this point wait for them to open up and if this is a man attacking you a quick kick to the groin will be more than enough to stop them.
If worse comes to worse and you have been grabbed don't worry even if the attacker is much stronger than you chances are that they won't have had any skilled training and this is where you are going to win and break free.
At this point look for any soft tissue areas, the groin, eyes, neck, ears or cheek are vulnerable on anyone's body no matter how big they are.
Punch, kick, slap and even bite any of these soft tissue areas and while you're at it scream, growl or make any noise you want to, this will cause them to think you're not sane in the head.
Combine all these elements and you will be able to slip away even from the biggest of attackers.