if you're fat don't wear short tight clothes... i dunno, pretty much the same rules for girls as for guys. Try not to be one of those cross-dressers who is all ridiculous. If you wouldn't want to see it on a girl, don't wear it. also no pastel colors (unless its awesome).
look at what cutely dressed girls are wearing and try to buy one like that. Personally I usually wear somewhat form-fitting skirts, but that might not work on you unless you aren't fat or muscular.
Don't go to thrift stores and buy clothes from the 80's, either.
Pretty much don't be tacky
