Since we have to check our bags when we travel anyway, we'll put the clothes for all three of us in one suitcase. If it's over the airline's weight limit, we'll split it into a smaller bag as well.
Finally, souvenirs...if we can't fit them in luggage we'll do one of two things. Rethink the purchase...do we really need it? Where will we put it? Is it worth the hassle? The other option is to ship it back home. Most places have businesses such as Kinko's or the UPS Store that will do this for you at a nominal fee.