I follow a similar routine as peebee.
I dump the used potting mix and from there I treat it like a compost pile. I add food scraps, leaves and non-diseased garden waste in it, turn it and add water. I am not very vigilant with making sure the the "pile" is hot or that I turn it and water it at proper intervals. Currently I store the used stuff in a giant bag, it is not a construction bag but close to it and a large garbage can (the curbside type) that I drilled holes in.
Then when I use it I tend to use it as the bottom layer in pots.
If it is a plant that does not require high quality growing medium, I may fill the entire container with the re-used medium.
At this time I only have one raised bed and I also mix the re-used stuff with the native soil.
Sometimes I sift the mix, I use a
kitty litter sifter.
If you want to use all of the potting mix from a previous season I would follow what akgardengirl stated which is to amend the used potting mix with quality compost and, if needed, add perlite for drainage aeration.