I'm always on the lookout to find cool little tricks that will help turn on the nervous system and allow for a greater workout. One such method is doing an isometric off of a swiss ball to strengthen the neck musculature.
Sit on a Swiss ball. Walk forward until only the back of your head is supported on the ball. Keep the hips up by contracting the glutes. So now the only points of contract are the feet and the head. Try to hold the neck in a neutral position for 60 seconds. If you feel yourself losing the neutral position, stop the set.
Activating the long cervical extensors can help reposition C5 and C6 (two vertebrae in your neck) which inervate the biceps and a few other muscles. This will increase curling and pulling strength (it may increase biceps strength by as much as 10% according to Charles Poliquin). So next time your head off for a set of pullups, try this neck exercise and see if it doesn't have you "pulling" a few more reps.