Sorry to contradict the last poster, but you do pull manes to shorten and thin them.. I hate cutting manes since they look so unnatural.. pulling manes leaves them looking natural, and you don't have to worry about getting it uneven as with scissors.
It's a short metal comb that you can get at your tack store, like this one: http://greenhawk.net/cgi-local/SoftC...d86+1201153486
You grab a section of mane, backcomb it until you're at the bottom-most layer thin enough that you can wrap it around the comb and pull out, and do just that.
Do this until you are at the desired length when you brush the mane out.