I think Sneak Attack should be made a Toggle. Turn it on, then make an attack. Turns off if the attack hits. That also allows you to mess around with things that would proc sneak attack and get it on the target or attack you want.
It's important that a Sneak Attack Toggle stays on after a successful sneak attack. This way, you don't have to manually turn the toggle on every single turn and it automatically allows opportunity attacks to deal sneak attack damage. Obviously any additional attacks during your turn wouldn't deal sneak attack damage.
