There are some good posts out there on healing. However, I will share some things that I do.
Do not feel like you must keep the tank and DPS at 100%. I usually keep the tank around 80-90% and DPS around 50-60% health. There are a couple of reasons for this, you want to try and avoid over-healing and you want to delay landing a heal for as long as is comfortable (5 second rule) to generate mana back.
5 second rule encase you are unfamiliar, you do not gain mana while in combat (until later when you pick up some talents and some mps gear), this means unless you pot, you’ll go oom. However, if you do not cast a spell for 5 seconds you start getting mana back. So, the mata game becomes “how long can I delay healing”, sadly the answer to that depends on how confident you are and the group.
Pre-casting is super important, especially at lower levels. The idea is simple, queue up a heal or greater heal on your tank then when the cast is almost done decide whether to let the heal go through or cancel it (I usually just strafe left or right to cancel the cast).
Deciding to let the heal go or cancel. I usually use something like HealBot or a UI that shows ‘incoming heal’ and will cancel the cast if there isn’t a small gap left after healing (to avoid over healing). Most of your mana efficient heals will have long cast times, pre-casting makings that a non-issue. Note, the 5 second rule is only broken when a cast has been completed, this includes instant casts.
Side note: Usually, before getting a group I’ll sit in Org or UC and ask for mage water (saves a lot of money) and I’ll start with minimum 40 drinks. After each pull, run slightly ahead and drink as the tank pulls. This will help you keep your mana up without slowing down the group. (typically, I’ll throw a renew on everyone just before i drink).
Once you’ve mastered the 5 second rule and pre-casting, you’ll find yourself sitting on 100% mana quiet often. This is when I would start considering the usage of renew on the tank, which will allow you to not have to cast again for a while.
Never use flash heal or shield. Both spells cost a crap ton of mana and will leave you oom quickly.
Hope this helps.
Edit:
Just to add, if you find yourself needing an Oh-crap heal on the tank. Renew followed directly by a greater heal works 9-10 times. renew should keep him up just long enough to land the long casting heal and this will leave you enough mana to keep on healing afterwards.