Since my boyfriend installed a pull-up bar at home (with three grip positions), I can do it randomly throughout the day. That makes a huge difference. At first I could only lower myself but now I can partially lift, this is after only 2 months... I didn't even like to try at the gym because I couldn't do it at all and didn't want to look totally lame.:p

Chuck Poliquin has a good blog post on chins/pulls, his tip on a beginner's progression starts about halfway down the post:

Chin-ups: The King of Upper Body Exercises

Of course you can do it!

I think your belief about whether you can do it or not can make a difference. Women especially are influenced by what they belief about their own body strength. It took me about 4 months of focused & really slow negatives twice a week to finally do one.

Then one day I was at the park & without thinking about it, I did a perfect slow pull-up from a dead hang. Once I got out of my way & knew I could do it, it was easy.

Personal Trainer (and Primal enthusiast) Al Kavadlo has a great e-book that may help. It's called "Raising the bar & is n his website: http://www.alkavadlo.com/

I can tell u from my own experience I never got one until I dropped the weight. I would work at negative reps, use bands, and other lifting to target the muscles needed but it wasn't until about my 4th month and having dropped 50 lbs before I started to getem. I can do 20 in one set now and work in 5 sets twice a week. Weight for me was the big issue, that's why I believe in eating paleo. I've cut 90lbs since this time last year and am finally to the point my weight just maintains itself.

I could only do two when I got my pull up bar in April, since then I try to just do as many as I can at once when I walk by it then continue on with my day. Now I can do several sets of ten pull-ups, chin-ups, and hammer-grips. This was before I started Paleo, so I don't necessarily think it's just losing weight as Bob Smiley mentioned. Just condition those muscle groups.

I read a pretty infrormational study on how body type affects this. Long arms make it more difficult, for example. There were other physical variables.

The basic conclusion was that anybody can train for and learn pull-ups.

The study fully explains why my flabby roommate can bang out a dozen, says he's always been able to. Meanwhile I've been struggling to hit three after months of working on them, and why I couldn't even do one when I started.

Steve Kamb has a pretty good guide.

http://www.nerdfitness.com/blog/2011/04/25/do-a-pull-up/#more-9278

Weight will be your biggest issue, but I think the negatives will be your best help. There are a few ways to do these, but I like to stand on a chair to the up position, then let myself down very slowly. I do this on random days to switch up a pull up workout, and it works great

Mark Sisson's video is what helped me.

6 months ago I couldn't do one pull-up without assistance, now I can do 4-5 in a row, or I can do 12-15 within 10 minutes or so if I taper them down to one at a time. My first month of bi-weekly pull-up workouts were focused on assisted or reverse pull-ups before I even tried to do a regular pull-up again. The main thing is it does take time, so keep at the assisted/negative pull-ups.

They really do work, though. I now have abs and oblique muscles that I've never had in my life just from doing moderate pull-up routines at a nearby playground for the last six months. That may be part of why it's slow-going for some of us, too, the fact that we just have underdeveloped pull-up muscles to begin with!

Keep at them, you'll get there.

Negatives, partials (from the bottom and the top) and assisted versions should all help.

You may be well on your way already. You can check out Primal Blueprint, Convict Conditioning etc. for progression plans that might help as well.

Just keep doing them.even if you can't your muscles will build memory.they will get stronger.then you can probably do one!