Here is the secret on this topic IMHO -
Choose a starting point - it doesn't matter where, thong, silk - WHATEVER, just choose something. Use them for a while - then CHANGE IT UP like you do with weights!! THe key is to keep things "interesting" in bed. Once a theme gets "Stale" you need to move on and not do that theme again for a while (till what is old becomes new again). Undies are in a sense just another sexual theme, and humans get sick of the same thing if you get it enough.