We developed a new methodology for determining charge concentration dependent mobility from organic field-effect transistor (OFET) characteristics, applicable for semiconducting polymers with structural and energy disorder. We show that basic formulae recommended by the "IEEE Standard for Test Methods for the Characterization of Organic Transistors and Materials" for the determination of the field-effect mobility as obtained from the slope ISD1/2vs. VSG (in the saturation regime) or from the transconductance dISD/dVSG (in the linear regime) are not suitable for materials with concentration dependent charge carrier mobility. We propose alternative expressions, which can be directly analytically derived from the drift-diffusion equation with the mobility explicitly dependent on the charge concentration. This methodology for mobility determination was used for analysis of the experimental data obtained for a poly(3-hexylthiophene)-based OFET with the bottom gate-bottom SD electrode configuration.