Dick Friel has posted an article on how to train for climbing if you don't have any hills:

http://www.active.com/story.cfm?story_id=12755

He describes two workouts. The first one builds leg strength, the second one builds leg endurance.

I suggest one modification to the workouts: if you're doing them on a trainer, raise the front wheel by 3-4 inches to simulate the bike orientation when climbing.

One further comment: the strength workout will certainly help, but standing start sprints are more effective at building leg strength.