you have to retreat sooner than you think, esp if you're used to heroes with more hp. you will be tempted to chase a retreating character but odds are they are going to grab a healthpack you haven't gotten to yet (or the hack expired), going to get healed up by a teammate, or chase you into an ambush. if you can get a pick, get a pick but if the tide of the fight changes, tp out. communicate with your team about emp. don't emp unless your team is about to engage (i see a lot of EMPs go off and no one's around to take advantage of them, or the fight is already lost, people are dead and the remaining will die soon).
Also, hacking enemy healthpacks can help prevent their sombra (if they have one) from getting EMP as quickly, are up for your team during a team fight, and of course prevent red team from using them.