I remember when I met Tito Ortiz, I asked him what I could do to stop puking AFTER my fights (regardless of whether the fight lasts 10 seconds or 10 minutes) he looked at me, smiled and said, "Dude, I still do the same thing" or something along those lines.
I suggest reading "Wrestle Your Perfect Match", it's the only thing that has ever helped me.
Basically what I do is create an Ipod playlist that I will listen to ALL the time when I'm at the gym, running, or just in bed thinking about the fight. When I run through the fight in my head, I never envision it going perfectly for me because that's not realistic.
I try to see the fight going back and forth but I'm flowing and doing what I do best and making it work for me.
Before you compete, you should ALWAYS have already hit your first wall. Basically, roll, hit mitts, and pummel until you feel like you're tired. After this, put on your sweatshirt or whatever to keep the sweat rolling and stay loose.
While I'm waiting, I like to listen to the same music I listened to the entire time I was training and thinking about the fight. The exact playlist in the exact order. It gets me in the same frame of mind.
Also, something I do that I just picked up when I wrestled was hype myself to a "chewing gum cocky" frame of mind. What I mean is when you watched Micheal Jordan play basketball, he wasn't out there shucking and jiving and telling everybody how great he was. Instead, he was smacking his gum with a smirk that indicated he already knew he was the baddest mother in the land and if you don't agree, you're about to.
I keep saying to myself, "Damn, if I am nervous as good as I am, how nervous must the other guy be?!"
It just helps me mentally and is probably the only reason I didn't puke after my last fight.
Hope any of that helped.