Personally I use focus for my main.. so my standard healing macros look a bit like this:

/cast [target=focustarget, help] [target=focustargettarget, help] [] Lesser Healing Wave

focus: my main
focustarget: the enemy targeted by my main, or the friend targeted by my main
focustargettarget: the enemy's target, which is the one who is going to get hit


This is probably the closest thing you'll get to "auto healing". Usually you can just spam this button and it will keep your party at good health.
The only problem with this is with multiple mob pulls in heroics.. you might get split aggro, upon which your healing macros will not properly heal everyone, so in these situations you'll need to manually spot heal members. This is no biggie in normal instances and in pvp.. but in heroics you sometimes only have a few seconds to react before someone dies.. and there's slave target lag that comes into play here which can lead to disaster (slaves heal themselves or wrong target due to target information latency).