1) Dont trickle in aka dont go 1 after 1 and suicide while trying to defend, just wait in spawn for your teamates and then go as a group.
2) Switch heroes once you have used the ult if you feel it isnt working. Too many people are stubbornly using genji when it isnt working out for them, just switch champs m8.
3) Have a good teamcomp. 2supp 2tanks 2dps tends to work out more often than not, dont be stubborn with your pick and just switch m8