On recent Canons, the print head is removable - looks like the one on the S520 is as well (it's the tub that the ink tanks set in - the big lever on the right of the head carriage releases it). So take the carts out and clean around the outside of their ink ports with rubbing alcohol and dry. If you can't find a way to seal the ink ports air-tight, throw them away - I have some film that's used to put over your windows in the winter (basically a heavy grade of shrink-wrap) that I use to seal mine for storage when I have to. Remove the print head and run rubbing alcohol thru it manually until no more color comes out. Touch the face of the head (where the nozzles are) to a piece of coffee filter (lint-free) to get the last bit of fluid out of the nozzles. Let it dry thoroughly, and then wrap tightly in plastic wrap - it's now ready for storage. IOW, you don't want to leave ANY ink or fluid in the nozzles for long-term storage.
. I would consider just leaving them together and just do a couple of passes of the self-test once a week or so - don't need to have it hooked to the computer for this. Anything to keep the ink flowing thru. Unless you plan for really long storage...
. One of those vacuum sealed storage bags (you know, the ones "Seen on TV" - Space Bags is one brand name) would be ideal to store the whole printer in - with just a light vacuum to keep the humidity out. But any heavy plastic bag would probably do - just press as much air out as possible and seal tightly.
.bh.