I don't think it matters whether you are wearing smart clothes or not. The couple of times I've been upgraded I was just wearing the usual jeans and T-shirt, and if you look around business class you will find just as many people wearing casual clothes as you do wearing suits.
As Julie said, travelling with kids is normally the best way to guarantee you won't be offered an upgrade. You are more likely to get upgraded if there are just one or two of you. Airlines tend to only offer upgrades if they need to free up seats in the cabin that you've booked in (due to routine over-booking, which is why charter flights tend not to upgrade), and it is rarely more than a small number of seats that needs to be found.
I've always assumed that asking outright for an upgrade, even if you do it very politely, makes no difference at all (or might even have a negative effect depending on the temperament of the check-in agent). Airlines only upgrade if there is a good reason to, usually an operational reason, and if you are in the right place at the right time you might get lucky.
If I'm able to delay my flight I usually ask the check-in agent if the flight is over-sold, and if it is I will volunteer to be bumped to another flight if they need to free-up some seats. What seems to happen is that I get put on a list with the other people who volunteer - a couple of times this list has been used to bump me and once it was used to offer an upgrade to First. I think this is a better strategy than asking for a complimentary upgrade, but you do have to be flexible about changing your flight times at the last minute.
Regards
Rob