You'd treat it like you would for a cathederal ceiling. I'm guessing that you have 2x6 rafters which means that you would probably use an R-21 insullation (5½" thick). There are two very important things to consider if you go this route.
1.) Make sure you have space between the insullation and the roof sheeting. Usually about an half inch to an inch, or so is sufficient. Owens Corning makes a product called Raft-R-Mates that is nothing more than styrofoam material that staples in place to guarantee an air channel. However, it's fairly thick.
2.) Make sure that you have a ridge vent installed along with good soffit ventilation.
What happens is that an air current is created to keep the room properly cooled/heated. Air will enter in through the soffits and travel up through the space between the insullation and sheeting as it is heated. Then it exits out through the ridge vent.
Now, the trick is finding a proper thickness of insullation. Since 2x6's are actually 1½" x 5½", R-21 would probably work the best. R-19 is 6¼", making it too thick (strangly enough). The other option would be to use the Raft-R-Mates and R-13 which is 3½" thick for a total thickness of 5½" (Raft-R-Mates require about 2" of space). I think if it were me, I'd go with the R-21 though.
You can then install whatever ceiling material you can fit up in the area. Drywall is probably best, but may not fit into the space you have to work with.