My suggestion is to never overlook the importance of base layers. If you're wearing armor or anything bulky, it's nice to have a comfy layer that won't turn you into a sweat-ridden sponge. If you're wearing a simple outfit and/or going to be outside in cool weather, having a warmer base layer can save you from having to lug around a jacket.
Uniqlo is my favorite for either case: their Airism line for when you're expecting to get toasty, and Heattech for when you need to ward off obnoxious AC or a fall breeze. They also usually have a line of easily packable jackets, which can be nice for commuting to the con.
My other tip is to never overlook the importance of good socks. If you're wearing fancy shoes all day, you're going to want to protect your toes. Hobbling around with bandaged feet on Sunday is never fun.