When you're facing another rein you've got to learn to watch their movements, expect them to have their ult up and try and bait them into using it. If you have your ult ready, you can sort of see when they are going to hit a firestrike because they drop their shield which is a prime time to use it
Watch your back, be aware of your surroundings and for any pot holes any snipers like to pocket themselves in.
Don't get temped to hit a charge that will result you deserting your team for a few seconds, try and keep it close range. Over time you'll begin to naturally learn when to utilize it better because it really varies depends on the situation (who and were etc)
People don't lose accuracy scores anymore so they will nuke that shield without leaving it to others so it will go down faster now.
Shield Management is vital, you don't want to get caught having it destroyed so don't be afraid to drop back a little to let it recharge a for a few seconds before jumping back, if you can communicate it on the fly. Good teammates aware of game mechanics will be aware of this, if someone complains of you dropping your shield then you'll find that means your awareness is better than theres. Don't argue with them they'll eventually adapt and start dying less.
Know your healers capabilities. Which healers are they? if you've just got a Lucio solo healing then be aware you're going to have a lack of healing so don't take any risks, it's you're teams increased dps job to get the picks not yours. If you've got a Mercy and a Moira / Ana or something then you'll have a little more freedom with your fire strikes and charges, just don't get too cocky.