1. Figure out where the goal posts are, and start out by making choices that will allow your to reach them.
2. Take into account everything you possibly can. This includes driver thermal and mechanical limitations, off-axis as well as on-axis response, magnitude and phase of the impedance curve, the speaker's acoustic environment, amplifier characteristics, and everything you know about acoustics and psychoacoustics.
3. If possible make your own measurements and learn what they are telling you, and what their limitations are.
4. Once you really get into it, the drivers will tell you where they want to be crossed over, and with what slopes. Don't be locked into textbook approaches - for instance, if your drivers tell you they might like asymmetrical slopes, try it. Remember that in the real world, the net rolloff is driver rolloff + electrical rolloff.
5. Expect to go through many iterations on your way to getting it right. Even with a professional-quality measurement system and computer modeling program and years of experience, I have never gotten a crossover right on the first try.
6. In my opinion, don't worry too much about being true to the purist ideal. Effectiveness is the measure of truth. For instance you may have planned to cross over at 100 Hz, but by golly it sounds better crossed over at 500 Hz, well then do that.