Hold yourself static at the top of your last pullup for as long as you can manage, then do a super-slow negative like The Red Dragon suggested, and then do another static hold at the bottom while you hang. It will finish you off nicely and help your grip strength quite a lot.
I did them. I credit them with helping to build my weighted pullup strength.
I've also found that in training women to do pullups static pullups and negatives are the fastest route to getting them to being able to do a pullup on their own.