Of course, a reasonable match of sensitivities is a good starting point.
Ideally you would have both tweeter level and crossover frequency adjustable, since there is usually a lot of experimenting involved.
The most widely successful high-pass crossovers for supertweeters seem to be either second order at 8-10kHz, or first order at 15kHz of higher. Below that crossover, the tweeter interference pattern with the main system becomes too obvious.
In fact, if the tweeter is much more sensitive than the woofer, sometimes it works to do first order very high, like 50kHz or even more, using that to adjust the sensitivity. I suspect this works because the tweeter actually does roll off, at least in power, even if the on-axis response is extended.
In some cases, people have had better success aiming the supertweeter away from the listening position, so it fills the reverberant field without drawing attention to its location.
Many systems (but not all!) have enough treble rolloff that you don't need a lowpass filter on the main system. Note that if the main system has nasty treble (as opposed to a lack of treble) the supertweeter won't fix that, and you'll probably have to cross much lower to solve that problem. That's a whole 'nuther ballgame.
