The spectroscopic catalogue of white dwarf main-sequence (WDMS) binaries from the Sloan Digital Sky Survey (SDSS) is the largest and most homogeneous sample of compact binary stars currently known. However, because of selection effects, the current sample is strongly biased against systems containing cool white dwarfs and/or early-type companions, which are predicted to dominate the intrinsic population. In this study, we present colour selection criteria that combines optical (ugriz DR8 SDSS) plus infrared (yjhk DR9 UKIRT Infrared Sky Survey, JHK Two Micron All Sky Survey and/or W1W2 Wide-Field Infrared Survey Explorer) magnitudes to select 3419 photometric candidates of harbouring cool white dwarfs and/or dominant (M dwarf) companions. We demonstrate that 84 percent of our selected candidates are very likely genuine WDMS binaries, and that the white dwarf effective temperatures and secondary star spectral types of 71 percent of our selected sources are expected to be below <~10000-15000K, and concentrated at ~M2-3, respectively. We also present an updated version of the spectroscopic SDSS WDMS binary catalogue, which incorporates 47 new systems from SDSS DR8. The bulk of the DR8 spectroscopy is made up of main-sequence stars and red giants that were targeted as part of the Sloan Extension for Galactic Understanding and Exploration (SEGUE) Survey, therefore the number of new spectroscopic WDMS binaries in DR 8 is very small compared to previous SDSS data releases. Despite their low number, DR8 WDMS binaries are found to be dominated by systems containing cool white dwarfs and therefore represent an important addition to the spectroscopic sample. The updated SDSS DR8 spectroscopic catalogue of WDMS binaries consists of 2316 systems. We compare our updated catalogue with recently published lists of WDMS binaries and conclude that it currently represents the largest, most homogeneous and cleanest sample of spectroscopic WDMS binaries from SDSS.