Let characters grow.
Have really tight continuity.
Do a full, total clean-slate, unapologetic reboot every 15 years.
Invent an excuse for the previous continuity to keep existing and allow it's use ("OH MY GOD, THE MULTIVERSE IS ACTUALLY ROTATING AND EVERY 15 YEARS ANOTHER EARTH TAKES THE SPACE OF THE MAIN EARTH!!!!!!!!!!!" or something).
Have as a tradition that Barry Allen dies at year 7.5 of each universe (or whoever the main Flash at the starting point is).
There, everything, for everybody, forever.