I've had the same problems. Treestyler has the right idea. Here are some more thoughts ...
The pulley needs to be clean and slick. No sap, nothing sticky. I clean mine with naptha and wipe with a rag sprayed with silicone. Also, don't abuse the pulley on rocks, cement or whatever. The little nicks will get to be a problem.
Chose a configuration that lets the eye relax open. Wouter is right about using what Teuf calls the "swinging" configuration. If you need a choke on the stem, go with the "swinging wrapped", one round turn about the stem. The "choked" configuration works too but the pulley needs to be slick. Be prepared to shake the rope, back and forth widely, to enouurage the Pinto to worm its way through the eye. Sometimes you have to keep shaking for a ridiculously long time, but it will eventually get though. Finally, the "constricted" configuration is worst. I don't use it unless I'm staying within a few feet. If you need a close-in TIP choked and retreivable, a U-SAVER is a better choice. The pulley jumps easily through the steel ring.
Regardless of these quicks, nobady's getting my PS away from me.
oldfart