You can either:
Drill the end of the crack
-Sometimes this helps stop it from spreading, if you're not a hard hitter.
Welding
-Ain't so easy, usually the weld retempers that particular area, and the molecular tension within the cymbal is all affected. Can lead to brittleness.
Bolts
-This is what the famous Nodar Rode in NYC uses actually, you can see it in action on Brian Blade's old Ks.
Dremel and smooth out the crack
-This is common practice by hobbyists, works best, and seems to last. I haven't heard any cases of the crack spreading again.
Basically a dremel is kinda like a drill, but easier to hold in the hand, with a controllable speed drive. Try googling for 'dremel tool'.
If you decide to do this, make sure you use the slowest speed possible, and go slow. The key is to keep the temperature low so that it does not temper the metal. Some people dremel a little, cool it with water, then carry on. Bit by bit is the idea.
I have personally never done it, haven't had the 'fortune' to crack a cymbal. Yet
Some examples:
(photographs were not taken by me, copyright belongs to the respective owners.)