mRNA can be concentrated using an Amicon® Ultra 30 kDa or 100 kDa size exclusion filter. These are available from EMD Millipore in various sizes. Simply add the mRNA to the filter and spin for a short time (~3-5 min) at the recommended speed. mRNA will be retained in the top chamber. Spin in brief rounds of centrifugation until the desired concentration is reached.  Note that over-concentrating the mRNA could lead to precipitation. Carefully remove the mRNA from the top chamber using a pipette. Calculate the actual concentration of the mRNA using a spectrophotometer or a Nanodrop™ device.