Oh yeah, the rules. Don't overtighten, use only enough pressure to prevent the strings from buzzing against what becomes the "zero fret". Position the capo immediately behind the desired fret (but not touching it), not on top of it and not mid-way between the frets. Keep enough padding on the piece which touches the back of the neck so that the feel thereon remains smooth. The capo essentially does the job of your hand, so position it as you would your hand so you get the best intonation and tone.

Otherwise, trust your ears, and play. Mooh.