These days I generally believe that you should wear what the hell you like but I can clearly remember thinks from my youth that were an absolute no no.
My mum was horrified when I went out in a coat with my skirt hanging down a couple of inches from the bottom. (This was the 80's).
I would never have worn red and green together, whereas now I have a pair of bottle green trousers that I like with a dark red jumper and hope that I don't look too much like an extra from Elf.
Do you have any particular 'rules' that you stick to?