Try to avoid patterned garments as they will detract from your portrait - by keeping it simple the focus will be on you.
If possible bring white and black underwear, together with white, black or neutral coloured vest tops, trousers or a long flowing skirt or dress - even an oversized shirt (possibly a man's) will do nicely!
If you have a sheer top or dress that is flowing or loose fitting please bring it along as that can create a lovely portrait too!