I think it goes without saying that the biggest factor in MP usage is your group. Is the tank holding aggro? Is the dps taking avoidable damage/debuffs? Are you taking avoidable damage/debuffs? are you using CC or breaking CC early? Some boss fights seem like it's unavoidable to burn through mp quick, but if you are running out on trash pulls, there is most likely a problem with your group not performing properly. If you find yourself spamming Cure II or Medica or Esuna(all of which burn mp pretty quickly when spammed) try suggesting a different strategy to your group(which I know can be tough in a PUG) but that is most likely the problem, the fights are not being mechanically handled properly for your group make up(classes/jobs) or for what your tank is able to handle. The most frequent time I run out of mp is when the tank has no idea what they are doing, it makes all the difference to have a competent tank.