I came across this site recently:
http://www.coverwear.com/cap_sleeve.html
The sell "undershirts" that are really-long. I have four of them now and they have become wardrobe staples! Low-rise jeans seem to be the only ones that fit me correctly; but, (thanks to PCOS and my hairy back) I feel very uncomfortable when I bend over if any skin is showing.
So, I just wear one of these shirts underneath my top, tucked into my pants, and all is well! I am 5'8" with a long torso and they are still plenty long. They aren't very thick, which is good because I don't like unnecessary bulk.
They won't work for everyone though, because they only sell up to a size 2X. I wear a size 18 at Lane Bryant and the 2X shirts are a good size for me. They are pretty snug (I definately wouldn't wear one without another shirt over it); but, they do a good job at keeping my back covered!
Hope that helps someone!