Jay.... just hit it hard. Upper body - lower body.. all of it. Don't just focus on the chins. Incorparate then into your routine, if you can do them.
And do them as long as you can..... I haven't been able to do them in 20 years!
I totally agree with Bob here, and on back day what I suggest is that you do as many reps of chins as you can and once you fail (it doesn't matter if it's 1 rep or 15) try incorporating negatives for as many reps as you can. So set up some type of platform so you can start the negative at the top of the chin and then s-l-o-w-l-y lower yourself down (3-5 count) and keep at it until you just can't lower yourself under control for another rep. Rest for 90 seconds and then go at 'er again, 3-4 sets of these should do the trick and in a few weeks your chin up strength will increase to where you'll be able to get quite a number of full reps.
DO NOT USE THOSE PUSSY ANTI-GRAV CHIN MACINES!!!
Who knows maybe in a few months you'll be the chin up king of your gym!