It sucks but you can only mega evolve once. So even if your mega charizard faints (it probably wouldn't happen) you can't decide oh well I'll just evolve my blastoise into a mega blastoise...That would't happen...Just have a team of stage 1 and stage two Pokemon you'll be OP. Now I'm definitely not saying you should never mega evolve here are some quick tips and stuff.
fishermen sends out lvl 1 magikarp (don't evolve)
red sends out lvl 90 gyarados (wouldn't happen but evolve)
Champion sends out lvl 60 dragonite (evolve so hardcore)
bug catcher sends out level 3 weedle (unless your a level 1 magikarp and your whole non-virtual life is at stake don't evolve)
There is also many times in tournaments where you think you need to mega evolve there will always be another tournament don't waste anything on a lvl 99 magikarp and if your saying in the comments you already did to you I say ... and I hope you learned a mega evolution is NEVER worth a magikarp again unless your whole life is at stake and your a level 1 magikarp.