I'm sure this has been covered to death in a few threads but I just can't find them. I suck at searches.

I'd like to be able to do pull-ups but lack the strength to overcome gravity to do more than one rep on a good day. Most days I can get maybe half-way up. Any suggestions/advice?

I've tried doing pull-downs but they translate poorly to pull-ups, at least for me. I've done the machine assisted pull-ups but when I get past a certain point of decreasing assistance everything falls apart for me. It's something I've never been able to do and would like to be able to do several solid pull-ups.