I posted this to my guild forum, thought I could get some good ideas here as well. Go!
With the changes to downranking, potion use, and mana costs, mana management while healing is more important than ever. I thought we could have a thread where we all post our tips and tricks for good mana management. Here are the ones I know, please add!
1. Use your Inner Focus, Trinkets, and Shadowfiend early and often. With boss fights being what they are now, it seems doubtful we'll see a fight where you can use your Shadowfiend twice any time soon, but it still makes sense to pop these tools early in the fight, in case you can use them again. Obviously, don't use your Shadowfiend until your mana's gotten low enough for him to fill you up.
2. Use Inner Focus wisely to get out of the five second rule. Don't waste it on a low mana spell. If you're going to use it with a Gheal, count to 2 first if you have the time. Use it for the next spell after a Holy Concentration proc for maximum time outside the FSR.
3. Cancel Cast. When you aren't healing other party members, keep a Greater Heal in the pipeline at all times, but use stopcast or another interrupt to make sure you only land it on the tank when s/he needs it. This may help you get a tick outside the five second rule.
4. Flash Heal is your new downranked Gheal. Personally, I didn't use it much before, because the mana/healing ratio was poor. But now it's better than a big mana Gheal that you don't need, probably even with Serendipity. Of course, for Disc priests, Flash Heal has become important anyway.