Yup as Willaus0001 said, dont bail half way round, it will hurt.....bad. Even if your feet come out the straps (happened to me 1st few times) keep holding onto the boom. At worse you should end up in a water start position ready to sail away.
Another trick which helped me was closing your eyes once you are in the air about to sheet in. Sounds crazy i know but it means you dont see yourself being catapulted (in a controlled way

)towards the water, makes you less inclined to bail.
Also move your back hand down the boom so you really sheet in hard, and least but not least look over the back shoulder, this gets your head in the right direction for when you land.
Once you crack it you will wonder why it took you so long to do them, they are piss easy once you get past the fear factor.