Soraka will have mana problems in the early and in the late(bcs of the warmog).
In early, only W when you hit a Q(you will have to hit them, if not, you are ****ed up)
In late, if you are starting to have 40% mana left, just dont spam W, try hit some Q and then heal everyone(Q bonus when hit cant be applied 2 times to the same player, but you can apply it to your whole team if the cdr let you). If you are out of mana, just tell your team you CANT fight, you wont be able to do a thing.

This is my personal build since a season(yes, even when the censer was broken, i sometimes introduced censer, but not normally), I think that you dont need to increase the ammount of life you heal, you just need to survive(bcs the enemy team is tilted bcs of you and want your *** so hard), and the best way to do that is with this build.