You can do "on ceiling" with mounts like ImcLoud suggests or you can do "in ceiling." In general, in ceiling speakers don't sound as good as others, but sometimes aesthetic concerns overrule sound, and I would also say that it isn't as big of a factor for surrounds as it would be for fronts. A number of companies make decent in ceilings if you end up going that direction - I like Aperion for their price factor.
One question, do you have a center channel?