For me: top, socks, belt and steel-toed boots. I'll wear underwear only if it's cold or if I know I'm going to be around small children.

I also have a sporran. I'll always wear the sporran with my MacGregor, which I don't wear too often. I get more mileage out of the sporran with my casual kilts. The sporran has two clips on short chains that I use to attach it to the D-rings on certain kilts -- each of my Pittsburgh Kilts, my USAKilt and my AmeriKilt have D-rings. So I'll wear it if I need more storage space.