The only advice I can give (and I'm no expert) is to make sure your fit enough and dont spar for too long.

A few years back (when I was 14) I decided to challenge my friend to a 12 three minute round sparring match using roughly 75% of power in our punches. We both decided I won on points having knocked him down in one round but he demanded a rematch a few weeks later which we both decided he won. I never trained for these matches and I'll tell you something......I've never been so tired in my life! lol It really takes it out of you. You should have saw us trying to breathe during the breaks!

Back to my point...... make sure you think your fit enough to do it, and dont start off with 12 rounds! lol