Use a little salt and boil them for about 20 minutes.
Make sure they dry well, a couple of hours, before you put them back on your bass. Surpise, surpise, they sound like new again!
The new string sound doesn't last as long as real new strings but it's a lot cheaper.

This trick works only once or twice.

VERY IMPORTANT !
Never use the pot again for cooking, because for some brand strings,
the residue that comes off, might be toxic to humans.