I went out today, it was very marginal wind prob 10 - 12 knots. I was able to get going and started getting good speed. Then when I tacked the kite flew over my head and then dropped out of the sky. Is there any techniques to prevent this from happening?
hummm, I would say you don't send the kite strong enough on the other side...?
it happenned to me a few times when I tried tacking on my hydrofoil board in very very light winds (~5knots)... but never on my raceboard...
my suggestion would be to send the kite faster towards the other side during the tack, the kite probably overtakes you because it is staying too long above your head...
It use to happen to me when not using a race specific kite.
Just curious what kite are you using?
I'm using a 2012 17m edge on 27m lines. It has happened a few times and usually makes for a long swim home.
I'll try turn the kite faster onto the other tack, next time. 
I'll see if that works. Nothing worse that trying to relaunch a kite that has fallen into the water backwards!
check your back lines - maybe to long so over sheeting and stalling out kite in LOW winds
If lines OK and it happens - Pull on your de-power fast to 50% to depower if it happens and kite should recover about 10m to 20m above water and fly off up to 12 oclock. Also if you are a heavy rider ( oh to be 75kg ) this happens alot if you are 90+kg in very lit winds if you are pulling hard on bar after tacks / gybes etc in 4 to 8 knots ....
So the pull on depower fast is part of the game...
I would say just try sheeting in to bring the kite back in the wind window. Use the lift from sheeting in to help you be light during your tack. Also if you drive your board into the wind more before sending it (the board) in the new direction this should help from the kite outrunning you whilst also gaining you ground upwind.
I suspect it was less than 10knots to have this problem.
Try learning to down loop the kite
If it has flown forward in the window and not making any more progress where you want it to go throw in a quick downloop.
Once it is flying back in the opposite direction fly it back up and sheet out to help it get there quicker 
This gets it flying again so you can regain control.
If you take to long to make the decision it will just crash into the water, you have to keep it flying, even if its not quite what you want to do at least it keeps it in the air
This is a good technique for light winds as when you tack and dive the kite down it sometimes wont come back up, so the down loop gives it some speed, gets you on the plane and off you go, after some practice 
