I'm a year or so into reusing soil. My experience/approach:
Many folks don't like
FFOF/FF soils. I can say I've had better (Power Flower from NoCal is the rockin'est bagged soil I've ever used) but as far as commercial stuff...it's done me OK. Not a fan of the Roots stuff/the funky coco/perlite mixes. I just like something that looks like dirt...even if it might not be. But I digress......
In reusing soil...I simply keep more perlite, worm castings, guano, and trace minerals on hand... and add those back into the soil that I shake off from the rootballs after each grow. Mix it up... and it usually sits around a bit...unless there's a huge transplanting/up-potting going on/etc... then I'll just start right back in. My approach is to go light on the re-intro of any additives because everything we add along the way has the same elements in it and alot of organic matter hangs about quite awhile. The last thing I want is a soil that just is too hot from the start. I can always add in amendments/etc....later.
every-so-often i will simply buy a couple of new bags of soil and add those in as well.....cus after awhile it seems the mix gets more and more perlite heavy (or light..however you see it)
Not a science...not at all. I go on visuals from the last plants that were grown in the soil I'm workin with. Did they perform? Were their problems? If not...reuse. If so...trash it and start over.
Sumpthin else.... If you use perlite (I know seamaiden..i know)... try wetting it in the bag... and while it is damp......sprinkle/shovel (LOL) your microbe/benficial product....or yer kelp powder.. or whatever it is you add to the soil on the perlite and coat that shit up with it. >Whatever< it is will get in all those little fissures... and IMO.... it makes a difference in root mass/etc cus the microbes get spread out widely in the medium vs. just sprinkling it under the rootball/adding it to the dirt/etc...
just another thought/tip from a crazed mind .... YMMV with sanity levels and general givin a fuck... :D