Shoot a tongue flick?
Really, you just have to take about 50 shots, and hope you get lucky.
If you want a clear pic of a flicking tongue you will need good light and/or high ISO. A flash is also necessary because the strobe stops the motion of the tongue. I would first try direct sunlight, try to get an iso of 1/1000 or better or use a flash. And as others mentioned you will need to fire a lot of shots to catch it.