I agree with Snowtoy, what I do is tape two relays together, wire one and leave the second relay ready to be swapped without having to do anything but unplug one relay and plug into the next. This way you always know where the spare one is.
I've also done this with spare ignition computers/injector systems, I just tape them back to back and then reinstall them together. Of course I've never had to actually switch one out.
