Reuse old soil? YES!
I've reused my potting soil now for 2 growing seasons, I didn't go 100% reused for a "fresh" mix, but added 25% fresh peat and some new perlite. Now for what I do....

The first thing I do is let whatever plant is in the pot die off completely. This seems to remove a good portion of the live root system as well as much of the green foliage. Once that's done, I shake out as much of the soil as I can into a 5 gallon bucket that I keep on the side of the house. Any large clumps of roots I take out and toss in the garden, in the beds around the house, etc. I keep that bucket going until it's full.

When that bucket is full, I take that soil out and transfer it to large lasagna sized aluminum baking trays that I picked up at the local dollar store. Just to the edge of the top, not overfull or they take forever. I typically get 4 of these trays per 5 gallon bucket. I take those, 2 at a time, and bake them at 325 for 2-3 hours in the oven. It does smell like earth cooking here, but I actually like the smell. It's the pieces that fall off the tray into the oven and come back smelling later, that can be nasty (especially when it actually *is* lasagna in the oven).

When it's all baked off I dump the trays into a wheel-barrow that I keep by the house and let it cool. Once cool, I add 1 small bag of peat to 5 gallons of reused soil. I then add some perlite, perhaps 1/2 a bag to break up the mix a bit.

The first thing I've noticed, the longer the reused mix sits in the 5 gallon bucket, the "cleaner" it is at this point. I've settled in on around 4 weeks in the bucket before I bake it.

Next, the Ph is almost always off, tends to be on the low side and needs lime to bring it up. Mine typically registers around the 5.7 to 6.0 on my testers so I add lime to bring it up to ~6.8. If you're planning on rolling your own soil, regardless of the sources, test the Ph before you use it.

I also add 1/2 cup of slow release granules to the wheel-barrow mix (Osmocote) and have also ammended with bone meal and blood meal for higher fast acting N & P soils. One thing I have found is that reused soils the way I've made is low in calcium and magnesium so I augment with a good fish/kelp emulsion (or even a mix).

Use organic fertilizers, fish and kelps are by far the best I have used. The brand I use is Neptune's Harvest but I've heard that Earthjuice is really good as well. Get away from chemicals, they're not necessary and plants have a difficult time processing them. To give an idea on reuse numbers here. I just finished baking the rest of the containers I had from the summer and have ~90 gallons of fresh potting soil. For what cost me around $40 in peat and perlite (forget my time) I have perhaps $500 in potting soil at my disposal. Go to Walmart and pick up the $7.99 17 gallon plastic tubs, they're perfect for winter storage of what you've made. I really believe more people should reuse what they have instead of tossing it out. If you do toss it, by all means, please do so in a lawn refuse bag. At least it *might* be composted by your refuse company that way.