Being efficient at riding flat pedals doesn't happen over night. Riding flats takes practice and skills that you are not forced to learn if you're riding clipless. I ride both and I'm certainly a better technical rider because of riding flat pedals.

Here's a great in-depth article on Flat pedals from MTB'r and Fitness specialist James Wilson: http://www.bikejames.com/wp-content/...nManifesto.pdf

#1 tip for learning to ride flats is to stick with them for atleast 12 rides.