I would focus on not allowing your elbows to lock out at the bottom portion of the movement. Leave a slight bend in the elbows to maintain tension on the working muscles for starters.
You can also focus on doing negative chin ups. Pull yourself up to the top of the bar and then concentrate on the negative portion. Give your 15-20 seconds to get down to the bottom. It will increase your eccentric and concentric strength overall.
Look into buying some bands that provide weighted assistance (much like a Gravitron machine). You can loop the band over the top of the bar and stick a knee in the bottom portion. The bands come in different tensions. Start with one that provides 10 to 15 lbs worth of assistance and go from there. It should allow you to perform an extra 2-4 reps per set. Once you remove the bands and go back to a bodyweight chin up, you should see an increase in strength.
I hope those two protocols help.