For any number of spokes cross there are 4 groups of spokes (e.g. 4groups of 9 for 36 spokes) 2 on each side. Sheldon advises this order: right trailing, left trailing, right crossing, left crossing. I find it easier to do a whole side at a time (right trailing, right crossing, left trailing, right crossing). It is easier to spot your mistakes.
see:
http://www.bikewebsite.com/build.htm