When playing Support, at what time should I run Clairvoyance and when should I run Heal?

I don't mind playing support, I actually enjoy it most of the times. My question then is, in saying that one of my Summoner spells is already Flash for both situations, when should I use Clairvoyance and when should I use Heal?

If you are solo queuing, don't ever pick CV. Your team simply doesn't have the map awareness, or coordination to take advantage of the power of CV. CV is strong when your team can use it to find out where people are and kill them. In a solo que game, most of your teammates are busy doing there own thing and complaining because you didn't call MIA. Pick up heal then, to help your lane survive a little bit more. Its a good pick even on champions that do have heal (Soraka with heal + summoner heal + ultimate is very hard to take down).

In most games, the AD carry should be running heal. This is because it benefits them much more because when you are away, they still have a "heal bait" ability. And if you ever get zones away from them, they still have more survivability. Instead you should pick up exhust most of the time, because it extends the control you have over the game. An exhuast on kennen the second he ult can win you a team fight instead of getting your team aced. Plus, exhaust with its constant slow and damage reduction will help a lot more if you get caught while you are trying to save a ward.