I would avoid reusing soil if possible. Otherwise, it needs to be sterilized. People recommend actually baking the soil in the oven.

I've found that it depends on the size of the container. My smaller balcony-railing boxes (6" deep, 24" long) and the smaller pots tend to be more vulnerable to contamination, so I try to clean them thoroughly and fill them with new potting soil mix every year. The deeper pots seem to be okay if I mix half and half (or so) old and new (if they weren't buggy the previous year). One of our neighbours on the ground floor likes having our old soil to mix in his back yard garden, so it gets a second life!

Potting soil does not need to be resterilized. The only time sterile potting soil is absolutely necessary is when starting new seedlings. A reason not to reuse potting soil is that the last plant growing in it was diseased. The soil in your back yard is not sterilized nor is the plants you put in containers.
That is why I mix old potting soil with perlite and new potting soil. It keeps the mixture light.
I've been doing this for close to 20 years now and it works well and saves money.

I reuse it. However, I do so organically. And I don't jsut fertilize it with a liquid fertilizer. After so long, the soil becomes a bit "drained" and not as full of life, so I add compost to make the soil more healthy and get it going again. Also, one could mix a new bag with the old container dirt. Thats helps to get it fluffy again too!

:)

I reuse it , too. never had problem. if it's too spent or the pot was really filled up with roots, of course i throw them away. i mix the used one to fresh one which is much larger amount kind of as a filler. i mostly grow annuals so it doesn't matter much. mix some compost, too to make up for any thing that might lack.