Chocolate foot at 1pm.
Big chain ring.
Third or fourth gear from the top of your cassette - smash it.
Practise, practise, practise.
Anticipate the start signal..
Good luck.
What MarkyMark wrote is all you need to know. Starts for XCO and XCM are the same (if you want good position on the start loop, or to hit the single-track in a non conga-line formation/stay with the front group for XCM)
Dont worry about the first hill, just take off for 15-20 seconds, get a solid position in the group and then settle into YOUR tempo. If you are strong in single-track, you want to be in a good position coming into your strength.
Also at your different events, try to do a pre ride, to find out what the start is going to be like, and how far/long until the first single-track or climb. This will tell you (depending on your strengths) where you need to be on the start line, and how hard to go at the start.
Practise your gear selection on a flat road. If your gear is really easy to get going, but then you end up spinning in 10m - TOO EASY. If you are labouring to rotate your cranks from the get go - TOO HARD. Find that happy medium for you, its very personal.
I am sure that some others will get on here after me, and tell you "it doesn't matter", but if you want to do well, you dont want to be starting behind the eight ball from the gun.