MDW - 1/05/2012 8:20 AM 
My save a dive kit for shore diving (rivers, lakes, quarries, caves) is to have an extra of everything. This is feasible for shore diving because it all fits in the bed of my truck. For example, if I plan to do a long dive on doubles in a dry suit, I will actually pack an extra set of tanks, regs, BP, wing, fins, and a wetsuit in the truck in addition to the usual backup mask, spare reel, and tools and parts that make up a traditional SAD kit. If I am planning to do 3 single tank dives in a day, I may take a fourth tank, extra suit, fins, etc. This has come in handy on several occasions when I decided to do an extra dive, discovered a tank drained in transit, ripped a seal on my suit, or when one of my buddies forgot some gear or needed an extra tank to do an additional dive with me.
As for boat dives, I like to set up separate sets of tanks for the 2 dives with separate wings and BP attached to each. I’ll usually just take 1 set of regs and move it from tank to tank between dives. This is mainly to reduce the work I need to do between dives (avoid seasickness), more than for the redundancy of the extra gear, but it does also allow me to use that extra stuff on both setups if something fails on one.
Now, when traveling, none of this is really feasible due to space and weight constraints, so my core SAD kit consists of an extra 2nd stage with an adapter to fit a LP inflater hose, and LP inflater hose (can be used as spare inflater or with spare 2nd stage), spare mask (prescription - can’t see without it), multi-tool, DAN card, tables, a few O-rings, extra SPG, deco tables (air, 32, 36 & 40), and some bungee.
I just got a new DR Nitek Trio, so my old Nitek Duo will now become a backup computer, which I will have set in gauge mode so I don’t have to wear it on all dives but can switch to it and tables if the Trio dies (replaces analog depth gauge in SAD kit). I may also choose to always take it on dives with me so I can whip it out mid-dive if the primary fails.