A lot of it could be achieved in post, if you just want parts of the clothing to look wet. In the case of wet jeans shorts, you simply darken the color where it got wet. If you are going for a T-shirt, you would follow Shane's advice for that.

I did wet clothing for an image I put in my gallery if you want to take a look at it. Everything I did for that image is explained there.

http://www.renderosity.com/mod/gallery/index.php?image_id=2416326&user_id=663573&np&np

If I had a nickle for ever time a woman told me to get lost, I could buy Manhattan.