Do you workout at home or at the gym?
If you are struggling with your chinups I really recommend bands, if you don't have a person to push you up.
Check out this clip on youtube.
http://www.youtube.com/watch?v=EGBi2tNBPtQ
They are as close as you can get to the real thing. For the love of god don't use the assisted chinup machine!! I sooo HATE THOSE MACHINES.
If you go to a gym that doesnt have the bands briing your own, you will still get the most of the range of motion.
Oh one more thing you may need to do some supplemental grip work. One of the things that screwed me up was that I had smooth girly hands. If you can't hold onto the bar you wont be able to do a chinup even if you are strong enough.
Hope that helps.