I really don't spend a lot of time healing (it's my absolute least favorite thing to do in PvP), but this is what little advice I can offer you:
  • 1. Use your Heals over Time.

    HoT's will make your life a lot easier, in Frontlines in particular. They will save you from dealing with splash dmg, and basically auto heal anyone who is not being directly focused by the enemy. Having your HoT's up will buy you the time you need to focus on the higher damage targets.

  • 2. Buff your teammates whenever possible.

    In between fights, it's a good idea to reapply any buffs you can to ease the burden of healing when the combat starts. In small skirmishes, you will have given yourself a big advantage. In longer fights, you'll have conserved a lot of Mp. It also gives your melee's the advantage they need to close gaps on ranged opponents. They'll be able to keep the pressure up a little longer before they need to retreat back to you for heals.

  • 3. Triage.

    Sometimes you have to decide who you can save and who you can't. Act accordingly. Prioritize your targets. Battle highs/fevers, in particular, are worth saving, and any target who is being (or about to be) gang killed requires priority healing. Depending on what's happening in the field, you can anticipate the dmg. Stuns and status effects are a good indicator of someone who is about to take a lot of dmg, and melee's who dip in too deeply into enemy lines will likely become focused targets.

    Sometimes it is also necessary to let someone die. This is much rarer, but sometimes a target will get so much aggro that they are actually distracting the majority of the enemy and buying time for your team to retreat or re-position. Tank's, in particular, tend to be the ones who do this, because they can survive longer without heals. If you see a MASSIVE group of enemies ganking a tank, and that Tank is using WD or HG or Holmgang, you have to decide if it's worth trying to save them or not. Sometimes it's better to let them sponge it all up and take a hit for the team, to buy the rest of the team time to escape. It's a pull out tactic that is sometimes used to avoid being chased by the enemy (which sometimes works and sometimes doesn't, depending on how fast your teammates run away). You can try to save them, but you could very well end up dying with them.

    Keep in mind that it is pretty rare for a Tank to act this selflessly, so you probably won't have to deal with this kind of circumstance often.

  • 4. POSITIONING IS KEY.

    The party does not always stick together for AoE heals, so it can be a real pain to keep everyone up. If you find your teammates are scattering a little too much, sometimes it's good to remind them that they will not get heals if they do not stay in range.

    The next thing is to keep yourself in a good spot. This can be tricky. You have to stay close enough to the front line to keep your melee's in range, but you don't want to be so close that the enemy team can easily get to you. Try to hide behind the bulk of your teammates. That means constantly re-positioning to keep yourself safe. Keep in mind that you are a priority target for enemies, so the harder you are to get to, the better.

    It's also very important to stay aware of your surroundings (more so now than ever before) and keep Sprint at the ready. It's easy as a healer to get tunnel vision on the party HP list. That's a good way to die. If you're not paying attention to what's going on around you, you can suddenly find yourself surrounded by enemies. Healers are RANGED players. There's really no excuse for not staying as far away from the enemy as possible. You're going to want to learn how to keep your eyes open quickly, especially with the upcoming changes.

    They recently announced that Sprint will no longer cost TP in PvP, as of patch 3.2. A lot of casters are crying about it, because they are losing their crutch for easy escapes, but, to be frank, if you're letting the melee get that close without them being punished for it then it's no one's fault but your own. Experienced PvP'ers always knew how to manage their Tp for a sprint anyway, so this change is not actually more than window dressing. All it means is that you will have to be VERY aware of your surroundings and act quickly. If you are keeping a good position, then you won't have to worry about it most of the time. Any attacking melees should have to get through your team to even reach you. Obviously, accidents happen and melee can sometimes close the distance, but that's where Sleeps, Binds, and knock backs come in handy. You have a TON of tools to keep melees away from you and should always be able to outrun them. The end goal is to avoid being stun locked and ganked.

  • 5. Use your PvP skills and set up a PvP hotbar.

    If you plan on PvP'ing regularly, having a PvP hotbar will make a world of difference. PvP skills, as well, will save you from a lot of trouble.

This is about the best I got. More experienced healers than myself will probably have better things to add, or correct any mistakes I've made. Most of it will come down to first hand experience, though. Welcome to PvP, and good luck out there.