Rate of drying is indeed the very important parameter. In order to prepare a very uniform film, first deposits the solution on a substrate. Then, place the coated substrate with cover (such as flask). Also, place a separate container with pure solvent used right besides the sample under the save cover so that the vapor from the pure solvent near saturates the environment. This will substantially reduce the evaporation rate of the solvent from your film. This also reduces the surface tension effect so that film quality becomes homogeneous.