Reviving dead strings
A well-known trick for electric bass and guitar is to boil your strings to get all the gunk out. Provided the string isn't completely rusted out (and isn't plain steel, obviously) it immediately brings back the upper harmonics and gets rid of that dead sound. Bass strings can last a decade with this treatment!
Well, maybe not. But it is a huge money saver.
Anyways. I was wondering if anyone has tried this with synthetic core strings, 'cause I have a real nasty Larsen A, a crusty old dominant D and some long-past-their-date Evahs I keep as spares (with my sweat, you never know when a string might pack its bags and run for the border). They sound awful, though. It'd be nice to have spares that at least sound passable in case of emergency.
A fun factoid: if your E string breaks and you're really in a pickle, nylon fishing line works great. By great, of course, I mean it's shrill and horrible and slippery and just bad. But if you're considering using fishing line as a spare string, tone is probably not your first concern.
Bury your long-gone cadavers, and keep buying new strings, would be my advice. It's nice to have a few not-so-dead around in case of string breakage, but not worth pondering whether one can re-use dead ones.
There isn't much you can do to revive the synthetic core strings. You can prolong their life by diligently wiping them down after every session and removing any rosin buildup with a little alcohol, don't get any on the instrument. Also a bit of 0000 steel wool can be used not too aggressively. A set of Pirastro strings should last 4 to 8 month and Infeld 6-12 months with diligent cleaning. If you are looking for decent sounding spares, just change a set a little early and keep those around just in case.
A plain gut string that has had its day will have a useful after-life in a garden.
Life is too short for playing with old strings and bad instruments.
I learned from my father to keep old strings in my violin case to help out orchestral colleagues when their strings broke (helped myself a few times too). Being a sucker for trying new strings when they were marketed, it seems I was always removing my old strings while they still had some life and I would keep them as those "spares."