I wear fleece pj pants and a cotton t shirt for tent time. I hear denim is also good to wear but I wouldn't want to wear anything that might catch their nails...you know, like lace. I'm very careful not to wear a lace bra just in case that would be the day I get a bra baby for the first time. Certainly don't want any negative associations with their experience, you know?
I also make sure to wear socks because, as my husband discovered, NOT wearing socks means the toes get chomped on.

Poor guy!