Pullups are flat-out hard as hell. Here's how to really pull your weight:
1) Elastic bands
Use elastic assistance bands for a boost. That'll help get you to knock out a few more reps than you otherwise would.
2) Negative Pullups
Focus on the lowering portion rather than the lift. Find a low bar and jump up so your chest in near bar level, then slowly lower yourself.
3) Forced Reps
When your muscles are on fire, have a training partner give you a boost to crank out two to three more reps.