Depends on what you're planning to do. The forearm thing is true :D I'm not a butt person, but I know many girls that are. Don't wear jeans that are toot tight. If you'Re going somewhere nice, suit up. If it's somewhere inbetween formal and casual, i'd go with jeans or chinos, a shirt and a blazer. If you're more of a chilled person and don't like to dress fancy, just go with jeans and t-shirt, a hoddie if it's cold. It can look quite hot on some guys :)
Personally, I LOVE a man in a suite. It's the most hot piece of cloth a man can wear. If it's not formal, then let your jacket home, but if you get a nice black shirt and some fine trousers, man, you'll make her go crazy.