When they’ve got to go, they’ve got to go! I’m not sure it’s possible to prolong the life. Someone (Carl?) may have some tips about humidity, heat etc that could help stop them breaking prematurely but I too just keep a spare set and change them when they ping. On the plus side, in over 25 years, I’ve never been hit by a breaking string!
(On a slightly different track, I seem to have two harps with an “anything you can do, I can do better” mentality. When one breaks a string, the other will also break one within a day or two. Spooky! Anyone else find this?)