The reason the airline has the rule isn't just weight, but is space as well. It's easier to find space for two smaller bags in a luggage bin that weigh 30 pounds than one overstuffed monster weighing 60. At any rate, it is moot - you shouldn't be packing more than 7 days' worth of clothing anyway (see #3).
My advice for getting around the baggage restrictions:
1. Get a very good carryon for both you and everyone else you are traveling with. Remember, you get one carryon and a 'personal item', so ladies, get that huge purse ready.

You should ALWAYS have 2 days worth of clothes in a carry on just in case the airline loses your luggage. If you are making a connection through one of the larger hubs (like O'Hare, DFW, or Hartsfield) then this is a MUST.
If you travel a lot, I highly recommend the TravelPro Rollaboard or something similar. It will pay for itself. I have one that is expandable and has a compartment for clothes that would normally need a hanging bag. My mother - who always needs separate shoes for each outfit - is able to put 4-5 days worth of clothes into hers.
2. This one has already been covered, but ship your stuff home if you buy a lot of souveniers. I once went all the way to Australia and there was NO WAY I was going to fit all of the crap I bought into my suitcase (by the way, I didn't check a single bag for a two week trip). Qantas allows two carryon bags so I had a rollaboard and a backpack. I was able to pack my goodies but gifts went via Australian Mail.
3. If you are going for longer than 7 days, then plan to do a load of laundry or two. Plan to have some morning or evening just to do laundry. I never pack for more than 5 or 6 days. If I'm staying longer I just plan to wash clothes while I'm there. This may also have an impact on where you choose to stay - you may want to find a place that has a washer and dryer or easy acccess to laundry facilities. However, if you are already going for that long you are probably looking for places with a kitchen and other facilities. Why not find a place with a washer and dryer as well? Most condos will have them already.