Well in the 70 levels I spent as a balance druid before speccing to resto I found a few things out.
One, the optimal spell rotation for me was to open with Starfire, Moonfire and then Wrath. Rinse Repeat. I also found spamming Moonfire was not a good idea, nor was spamming wrath both of which = oom.
Basically the way to roll through your spells is to do this. If you're doing instances and raids when you pull trash it's ok to spam wrath without going oom, which shouldn't happen if you've got dreamstate/intensity. When you're going up against bosses and mana isn't much of an issue go with the spell rotation I previously stated. If it's going to be a longer fight use Starfire and keep up moonfire/insect swarm damage. IS is the most amazing spell you can use for your tanks since it's going to cause them to be hit a percentage less.
Starfire is your most mana sensitive spell you have and it's got a pretty decent damage/mana ratio to it so use it well.
If you're going to be doing pvp, I would always Starfire, Moonfire, Entangling Roots, Insect Swarm, after that what it would depend on is if they had popped trinkets or cooldowns to remove themselves from roots I would reroot and begin to nuke with wrath.
Play around with different spell rotations, I know when I had about 650 spell damage and around 19% spell crit I could starfire/moonfire between 2000 to 4000 depending on crits. Really it'll just depend on what you find to be the best and most mana efficient way for you to kick out dps with your mana.