I see what i have and decide how I'm feeling that day, and how the items express my personality.Then I experiment, mixing colors and items, until I like what I see, and the impression it conveys. Sometimes I just happen to pick up an item that goes really well- for example, i just picked up the pawn as a present for a person who's no longer taking them, so i kept it, and found its tassels go really well with the color scheme I have in play. Lately I've been going darker and kinkier with my outfits. Sometimes also, if I pick up a new item, I'll try to build an outfit around it.
Occasionally the outfits reflect the time of year as well.. but I'm not as concerned about dressing "for the weather" as I used to be.