When is appropriate time to persist configuration?

[Tricky]

I have the following problem: I save(xml serialize) my configuration(very large list of object) on app exit, main form closing , but sometimes OS (win 7) on some comps terminates my app before the save is finished. So I'm asking when is the best time to do this , I thought maybe on main form minimize,but that to conditional(user maybe will never minimize the app).