My first piece of advice is rather flippant, but here goes anyway...
"Know the rules for your own army"
You can't expect your opponants to give you an easy ride, so know exactly what you can and will do in each game. Remember that Mawseeker means Stupidity tests every turn, remember to roll for the Wydstone Necklace and Gnoblar Bickers, remember what your stats are even after your buff spells are cast. If you can keep all that in your head then at least your side of the table should be ok.
My second point can really be considered a follow up to this...
"Don't let your opponant run the game"
If it comes to a point where you could almost be playing an intro game, in that your opponant is dictating the pace of the game and telling you what is going on, then they've won. I don't mean in the 'you should move that unit there' kinda running the game, it's more subtle than that. If you find that they are the ones saying, 'ok, combat phase. my I is higher so here are my attacks... that's ** wounds, now you get ** attacks hitting on *+, wounding on *+, now my saves, and I win by *' then they are dictating the game to you, rather than it being a two person thing. Take a break, stop and think for a bit and start being more assertive about your own army. See, relates to point 1!

Finally, as Ace said. Have fun and don't get downhearted if you lose all your games. Depending on the type of event it is, there may be many people there who have been tournament gaming for a long time, with some very mean lists. If you get through the weekend smiling and not complaining, then no matter the results of the games you can count yourself a winner.
(That being said, I've never managed losing 6 games and smiling through all of them. I usually got all upset round about game 4...)
Good luck!
The answer to everything is sabretusks. There is no problem that can't be solved at least in part with kitties.
Record: 17/14/5
Last game: 2400 vs Warriors - Win
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