Timing: yes its important, though it's not everything. If you look really really good and are off time in pre-bronze, there are judges who may assume it's a fluke and call you back for another chance in the next round. Waltzes with confusing melodies often have a fairly clear drum hit or piano chord going "_ 2 3" underneath, so you can listen for that. Also, go over to the 'music store' on this site and listen to lots of the preview clips and practice counting the timing.
Dress: be neat. Girls, make sure you can dance in it. Guys, do NOT wear a tux or suit jacket unless it was specially made by a dance tailor, a normal one will ruin your posture completely. You can look very sharp is a simple black vest or sweater over a white dress shirt and tie. Everyone who is not completely bald would benefit from hairspray or gel.
Presentation: Smile! I know it doesn't seem important, but it is. Also practice walking onto the floor together and bowing together at the end.
Winning is ultimately a matter of looking like you know what you are doing - even if you don't.
Figures: better to use the smallest number possible and do them well than to attempt lots of things with poor form.
Don't give up: I've been recalled to the next round even after bumping into a judge!
Food: Eat breaksfast, even if you don't normally (though your really should). Have something you can take in with you - energy bars or something, even if you have to smuggle it in and eat it when no one is looking - you don't want to stand around all morning waiting for your event to be called at any second and end up dancing sometime in the afternoon on an empty stomach.
Finally, don't worry - it will be over in 120 seconds or less!
