This has been a great thread! I've learned a lot from everybody.
Allegirl. I've been running with a thong for years under my running gear. No panty lines! As far as chaffing in the back, I've trial and error'd this one to death. What I've settled on is actually Men's Clever mesh thong. The wide semi non elastic strap keeps the junk in place with no chaffing in back, even on the long hot mid summer runs. I'm still trying to find a solution for friggin hammered bloody nipples after sweaty runs in the summer. Nipple scabs, calluses are just plain wrong. Humm... on second thought my feet look like runners feet.... argh!!!!
Three years ago I did switch to Merrell Barefoot Run Trail Glove, nice yellow color! For a year I did try the Vibram 5 finger running shoes, but after a few instances of snagging and breaking my little toe on various objects; I've settled on the Merrell shoes. There were a few unanticipated benefits from this switch, my skinny calves bulked up from running on the balls of my feet as opposed to the heels. And my knee pain went away and mileage went up! I would miss running if I had to give it up.....