Oh, Stacey. I had to smile when I read your post. It was so descriptive of my (former) life that it made me laugh!
We live in a three-story box house with five bedrooms (one used for storage, one for my office), one bath (very poor planning there), a parlor, dinning room, kitchen, and entryway. I used to spend the entire weekend cleaning. Then I'd get ripping mad because by Tuesday DH and the kids had trashed it again. But have heart! I found a solution! LOWER YOUR STANDARDS! OK. It's not very original but it works. Here are a few other tips:
1. Tackle one room per day but do not stop til you are done (then you can move on with a clear conscious).
2. Put toys and other items left where they don't belong by unthinking family members in a "time-out". Short of essentials (such as school books and business papers) this works for all items. Trust me -- after losing his box of Pokemon action figures for a week, my 6-year-old no longer leaves them spread all over the parlor floor.
3. Close the door. Your kids' rooms are disasters and you've got company coming. Close the door. Any mom or dad will understand that you can't keep your kids' rooms spotless. That is why they invented doors in the first place (bet you thought it was for privacy, huh?

).
4. Bargain with the kids (and DH, too). You want me to take you to the park so you can rollerblade. Gee, sorry. I've got too much housework to do. However, if you run this dust cloth around the parlor and bring all the dirty laundry to the laundry room, I might be able to spare an hour so you can skate. (This tip works especially well with my 13-year-old daughter who seems to think that "Mom" and "chaufer" mean the same thing.)
Hope this helps. If not, just remember that all the extra housework counts as exercise.