M.,
I have a 4 year old and last summer his room got pretty out of hand. He went through a phase where he didn't want to clean up when he was finished with something and we had a million toys scattered all over his room. Afer me cleaning the mess several times we finally came up with the following system:
I went to Target and bought several different size storage containers (mostly the clear ones). I seperated his toys into "themes", for example: duplos in one box; legos in one box; wooden train set in one; hot wheels and car roads in one; bigger size cars; balls; you get the idea...
My son knows he is only allowed to have one "theme" out at a time and he is only allowed to have his toys in his room. I started teaching him when he was very young to clean up (we even sang the clean up song when he was younger - it helped motivate him and make it fun) There are days when his toys don't get cleaned up before bed, like if he is in the middle of building a cool track for the cars I will let him leave it out. Because some of the themes are pretty extensive (we have a 50 gallon tote of the small size legos! and he has well over 100 hot wheels at this time)it does get out of hand sometimes, but at least it is always contained to his room. We live in a small apartment so I would go crazy tripping over toys everywhere.
Also- for his past birthday, when everyone was asking what to get him I informed them of our theme system and almost all of his presents fell into one of our current themes.
So far this system seems to be working for us. His room stays very tidy-- much cleaner than I remember mine to be when I was a kid. We praise him often when he cleans his room. He even makes his own bed now. He is very proud of his clean room.
I hope this helps. Good Luck with your toy situation.
A.