If it's really good soil, you should be able to reuse it two or three times with no trouble. To do that, just use a sharp knife to cut out a small part of the old root system making just enough room to plant the new plant. Use caution though, if the last plant had trouble in it or was less than robust, you might want to mix some new soil in with the old. If you do that, then it's usually recommended to replace the top 2 or 3 inches of soil as that's where most of the salts buildup is. I would reuse soil only if replacing it was a problem. Like sub said though, there's a lot of variables to recon with.
Happy growing.