buy a good quality one
I second that!
You can get junky ones that won't last and they'll be shorted (causing it to crank right out of the box)
Also, this may sound weird, but if your starter windings are starting to short and draw excessive current, it can arc the contacts when enguaged and cause them to 'weld' themselves closed, causing a continuous crank condition.
(This happened to my boss's explorer, he went through a couple of cheapies, a blue streak, and a motorcraft, and they would all stick randomly - even with the crank trigger disconnected - until he replaced the starter and the problem went away)