Any and everything (pretty much) that challenges my adulthood/emasculates me turns me on, but one of my personal favorite routines is when my girlfriend asks me to help pick out her outfit and help her get dressed for wherever she's going, with or without me. To tease me she'll pick out something extremely sexy, a thong or a short/tight skirt for example and mock me while I dress her, ask me how she looks, if I'm hard, how bad I want her, and so on, reminding me frequently that I'm lucky that she even lets me do this much.
Without fail I'm straining against the plastic of my diaper by the time we're done and feel totally submissive and inadequate.