I found the secret is to adjust the healing percentages in your healer's routine. Set them to cast their fastest heals constantly on any health loss and you'll do a lot better.
I modified my settings on my disc priest to not cast any group heals until around 10%, and set the biggest fastest heals to around 99-95% health. Mana is not an issue and I don't think I've ever seen the healer's mana drop below 80% on an fight.
One other thing that really helps is change your healer's loot settings in DB to never loot, so they never get too far away from the tank.
I've now done 2 more groups to 85 and after my initial group, I've had a lot less deaths. There's still a lot of damage that gets pumped out on some trash, but still less deaths. The most damage comes when you switch to a different expansion and your gear is lacking.