If you want to go really nuts, take a look at the Elsinore by Joe Rasmussen: http://www.customanalogue.com/elsinore/elsinore_index.htm

He does a great job of designing a crossover and phase-inverting the tweeter to get an excellent step response. beat mentions that this is something most speaker manufacturers fail to account for.

It's about as good as you're going to get for a DIY design and a whole associated design guide.