Nice underpants can be almost anything new. Its the old favorites that you have to be careful of (if you are out to impress, at least). The pair that are so comfortable that you wish you would have bought another pair when you got them at that Walmart clearance sale.
Wearing nice underpants is almost a guarentee that no one will see them. Its the days that you have lost hope and pulled out the blue-turned-greyish-white cotton ones that someone will see them. This is a horrid phenomena.
There are ways to get around it. For instance, if you are in a relationship, your are much more likely to get seen in the good ones (of course, the bad ones will get slipped in there too). The only for sure way to avoid this is to throw out the overwashed cotton undies unless you plan on saving them for the days you lock the door, open the Ben and Jerry's and refuse to see anyone.