I NEVER know what to wear (seriously, I can barely dress myself - no fashion sense WHAT-SO-EVER) - if I am going out with people who know how to do that, I make them dress me (my LDR FWB is really good at this, as is my crazy lesbian friend - they take clothes I own and make me look like some sexy fun person instead of a bag lady). Or I ask the boys what they want to see me in (MrS has clothing preferences...Dude is a fan of "all naked all the time").
I DO, however, have a few fall-back outfits. It's not that I look particularly great in them but that they are really, really ME and I feel confidant in them - I am of the opinion that confidant people look good no matter what they are wearing.
My go-to outfit when I am at a loss for random "going out" - my favorite hippie-skirt from college (mom has repaired it so many times...), thrift store Doc Martins, a clingy V-neck with tucks knit T-shirt from my sister and a comfy olive green zip-up hoody that I stole from MrS. My dress-up going out (theatre, fancy restaurants) is a grey/black long slit skirt and sweater deal with knee-high black boots that my FIL once told me I looked great in. My casual date/meeting people: the jeans that MrS tells me my butt looks great in and a purple knit top with just a hint of cleavage that flatters my (new) curves. (I also have several "little black dress" deals for "formal" but I usually can figure out when they are appropriate
I like clothes that dress up or down depending on how I wear my hair (up vs. down - I don't know how to "do" it), whether I am wearing makeup, and are defined by the jewelry I choose to go with them. I think that girl-clothes are easier in this regard - you can "get away" with stuff. Guys are suit/no suit, tie/no tie, collar/no collar - the fudge factor seems minimal.
PS. My highest heels that I can walk in are about 3 inches - and we better be sitting for 99% of the evening. I do have a "play" pair of mid-thigh stiletto boots - which are meant to get me from the living room to the bedroom - generally with someone carrying me over their shoulder...