This article is from the Car Audio FAQ, by Ian D. Bjorhovde (email@example.com) with numerous contributions by others.
Basically, this requires a degree of patience. The subwoofer should be
started off at about 100Hz and adjusted until you are happy with the
sound. Keep in mind that the higher the crossover point, the more
power the driver on the high-pass will be able to handle but raising
excessively may cause the low-pass driver to sound raspy or unnatural.
The idea here is to first make rough selections to protect the drivers
and then to fine tune crossover point selections to achieve optimum
fidelity. It's all a matter of what sounds good to you after that, but
remember that even *minute* changes in crossover frequency can make
dramatic differences in the way your system sounds and images.
As a rule, subs should be crossed over no higher than 120Hz, a 6 1/2
mid should be able to handle about 90 Hz, a 5 1/4" should be okay with
about 100Hz, a 4" - about 500Hz, and tweeters vary from about
3500-5000Hz. These points all assume the use of a 12dB/octave
crossover ... if you have a steeper roll-off a lower crossover point
may be chosen. Remember, these are not hard and fast rules but rather
a rule of thumb to help you get started (and so you don't blow up all
your speakers when you are setting your gains!).