I think rested exp only resets upon unlocking a new class. Even then, it's still a bug and I think it's already been reported. Haven't looked into it.

I personally have switched classes just to use my rested exp. I logged off with my MRD, and switched to MIN (since I think rested exp is best spent on DoL/DoH) and spent them, switched back to MRD to play and logged back off with it. Next day logged in, switched to MIN to spent the exp, and so on.