My two cents:
One: Pay attention to what’s behind your charge target.
-There are very few reasons you’ll want to charge deep into the enemy team. Ideally, you’ll want either a long charge back INTO the fight after spawning, or a short charge that attempts to pin an opponent against a wall. Either way, you want to end your charge close to your team, regardless if you hit or miss.
-There are exceptions. For example, you can charge an ulting Zen to push him away from his team, leaving his team vulnerable to whatever made him ult inthe first place. You can also charge a dangerous target away from your team, like a Nano-boosted enemy Rein.
Two: Avoid taking barrier damage or recharge delay unless you need to.
-While most people will quickly tell you not to hold your barrier up if you’re not protecting teammates, you also want to be mindful of the fact that your shield doesn’t recharge while it’s up or for two seconds after it drops. If the incoming damage it minimal, say a Reaper spamming from afar, ignore it. Bringing your barrier up and down to block minor damage will only wear down your barrier mitigating chip damage.
Three: Don’t stand still when you are holding your barrier up.
-This is the single biggest piece of advice I can offer. Unless you’re pocketing a Bastion, never stand in one place with your barrier deployed. Your barrier is not on a cooldown like Orisa. Chip damage will wear it down, and then your team is vulnerable. You should be either mitigating damage while retreating under heavy fire with your team, or pushing forward into their team to initiate a fight.
-Even if you’re facing a double-sniper on defense, and their team isn’t pushing, you still want to position yourself constantly to try and give your team better angles they can fire from safely. If you stay in one spot, your ranged DPS can’t adjust their position at all, so the enemy team can abuse LoS to push in close.
Four: When your barrier is close to going down, either push in or fall back.
-Rein can’t do much at range. And if your barrier is going down, there’s no reason to stand in the open. Either retreat behind cover to buy time for your barrier to recharge or force the enemy into YOUR range, or push into them and start swinging. The choice for this comes down to your gut feeling. “Can your team win a fight if it happens now.” If yes, go in. If no, back off.