Considering the Cornell potential V (r) = − 4αs 3r + br + c, we have revisited the Dalgarno's method of perturbation by incorporating two scales r short and r long as integration limit so that the perturbative procedure can be improved in a potential model. With the improved version of the wave function the ground state masses of the heavy-light mesons D, D s , B, B s and B c are computed. The slopes and curvatures of the form factors of semi-leptonic decays of heavy-light mesons in both HQET limit and finite mass limit are calculated and compared with the available data.