I second packing up toys into bins and rotating them out every few days. Also, even a three year old can help sort through their old toys. We have over-exuberant gift givers in the family, so the basic rule is "new toy in, old toy out," which we began implementing around three.
Even better, have a well-stocked craft closet or bin. When my youngest was around your son's age, we started to encourage him to make his own wants. You want a new track set for your trains? Here's an old rug, fabric paint, and a selection of boxes to build it. Yes, mommy will help. I didn't want to play trucks either, but I enjoyed helping him build his own playset to his exact specifications. The things he came up with! By five he was adept with a hot glue gun and was making Mars rovers from recycling bin detritus. Now at eight, he taught himself to solder with a kit so he could build his own robots. This kid doesn't ask for toys often, in fact there wasn't a single traditional toy on his Christmas list last year. Now, when he sees something he wants, he sits down and draws up plans to make it. Of course, visits to Radio Shack for supplies are just as pricey as Toys R' Us! He has a true DIY attitude and sharp problem solving skills that surpass that of many adults I know, which I credit heavily with us spoiling him with supplies instead of play sets.
We learned this lesson a little late with my older son. He is much more attached to stuff and likes to buy things. He prefers the lazy way out. Why fix a bike when you can get a new one at a garage sale for $40? Fortunately, he's a seasoned thrifter and likes to hunt out a bargain, and is willing to wait until the price is right.