I have only ever done a big mountain style comp on skis but my advice from doing skateboard comps in the past is this.
1) Work out who the judges are and what they are looking for, because if they are looking for the best technical tricks and you do a nollie tailpress then you are screwed (in terms of winning).
2) Hit the course a bunch of times.. Work out a solid, stylish, smooth run that you can do EVERY TIME. This is your first 'safety' run. Depending on the level of the competition, this may be good enough to do well or not, but by doing this you get some points on the board and even if you don't win, you didn't slam on both runs which always bums you (well me) out. Landing it will also give you a major confidence boost for run two and get rid of all those nerves.
3) Work out a second run in the same manner, with better tricks that you are less consistent on. You still have to be confident you could do this run at least 2 times in 3 (in my opinion). But also this is the run you'd hope to win with. You should try to find the best balance between having a hope in hell of sticking the whole run and doing all your best tricks to impress the judges. Throw this down, be super stoked if you make it, and you have a fallback anyway if you don't.
4) Go out and have a load of fun. I know the above 3 steps sound super serious but that is my advice on how to beat the nerves you'll be the most happy if you had a good time, are happy with your skiing and had a laugh. Basically enjoy yourself and good luck.
P.S. Don't watch the other riders (my opinion) if you are nervous. Seeing them throw down won't make you a better skier, but it might make you much more nervous if they are throwing down, depends on what you are like (this is true for me).
