You're doing it right.
A cull that doesn't use the thumb is Kostya Kimlat's RoadRunner cull but it only works when you're culling a number of cards one after the other (not a stack) because the first card you cull is done the same way like you do now.
If you're worried about the card "disappearing" then you can do a convincing control by aligning two cards together, showing only one of them and then culling it, that way it will look like nothing happened.
Here is a video of what I just explained:http://www.youtube.com/watch?v=iUyGuejLuuE
The guy in the video does it very bad but you get the idea