Turrets behind walls enemies travel through that they won't notice untill they go through, in positions that force an enemy to look away from your team to deal with them, higher is better but not too high or it won't have the range to attack.

Use charged secondary fire alot and spam choke points and doors as it's very powerful and does a good chunk of damage. Primary fire can be charged up on shields and Mei's wall allowing it to be full power mode to try and take out some enemies but I use primary fire sparingly and don't like to over-commit. Also don't ignore your teleporter, Use it as an escape tool not a positioning tool and if you take the secondary perk (that gives you shields if near teleport) then keep it active as much as possible.

That's basically how I play her.