I recently dumped out a few pots of workable used mix onto a tarp, mixed in a bunch of perlite, a few scoops of bagged manure and several handsful of time-release fertilizer and refilled the pots. I'll use those for tomatoes and annuals. Sometimes I also add coco coir, compost or worm castings, depending on what's handy. I think the important thing is to make sure the old stuff isn't beaten to dust and still has enough structure to drain well. Also, it's important to add fertilizer at regular intervals all growing season.
Many of my pots end up like Nancy's — completely solid with roots. I don't even try to reuse those.
