I gots tha asthma too:

1) Relax
2) Warm up well
3) Don't push too hard too early (I often go easier than I feel I "should" early on)
4) If you feel an attack coming on concentrate on breathing deeper, not faster.
5) Concentrate on the "exhale" part to avoid gasping (leads to tension / cramping and makes an attack worse)
6) As stated previously: Enjoy the race!

And of course at the top of the list:
See your doc for an inhaler and experiment with it during normal rides. I've figured out that I need two puffs at least 30 minutes prior to start time for best results.

Have fun!

kaNUK