Everything needs a home - a shelf, a bin, whatever - so that it *has* a place to be put away.
De-clutter. Do you really need 16 varieties of the same thing? Do the kids really play with ALL the toys? Donate what you can and chuck the rest.
Make it a habit. It'll take a while, especially for mom and dad, but once it becomes a habit it'll just be something you do without thinking. But start small and go in baby steps. Too much too fast and you'll feel overwhelmed and give up.
There are a few 'rules' that make things easier:
1. Hands full - this means that you don't walk from room to room with empty hands; always take something with you that needs to be put away.
2. If you take it out, you put it back when you're done - this goes for toys as well as mom and dads stuff (magazines, mail, etc)
3. We do 2 'pick-ups' per day: once before lunch and once before dinner. we make it a race to see who can pick up the most toys and MOM wins :o)
My girls are 4.5 and 3 (and 6 months, but she can't help yet) and they put away their folded laundry (though it often gets 'unfolded' in the proccess), take their dirty dishes to the counter by the dishwasher, take their dirty clothes to the hamper, pick up their toys and help where they can - unloaded unbreakables from the dishwasher, dusting, putting away things other than toys, and they love to help in the kitchen, etc. At this age though the 'helping' stuff is optional. But they love it so I rarely say no!