I ride flats (grip kings) and had a bike fit. My fitter spent time discussing the "q factor" and how to plant my foot in terms of angle to pedal and angle to calf. He actually taught me that I'd have a better experience moving my foot forward more on the pedals, not on the actual ball of my foot like clipless, and spent time educating me on how to use foot movement to deal with specific discomfort. The rest of the fit applies to any rider. Find someone who isn't going to blow you off because they think platform riders aren't real cyclists - a fitter with the right attitude will give you a great fitting!

Also, have them check for a difference in leg length which could have you sitting off center. You can use inserts inside your shoe instead of clipless shims.