I would look at a lot of games from the GM I'm playing if any are available. Both his wins and loses. Anaylzing each one carefully looking for strenghts and weakness'es.
Then wait to play him. I have only been in one such event so far..
If I open, e4... I would also look at a lot of Bobby Fischers matches to help prepare as well.
About opening...
Share your thoughts. How many times have you participated in those events? :)