This might seem funny, but healing is really about making priority. This is ussually what I do.
1) Do not let the tank die...
2) Do not die yourselve...
3) If DPS gets AoE damage heal them if rule 1 and 2 do not kick in...
4) If DPS gets aggro I let them die if it will not get the party or raid into any danger. DPS should watch their aggro. Otherwise they will never learn.
Easy as that.
