Having to wake the Sony Xperia S via the power button at the top of the handset can be a tad frustrating given its size. This is especially relevant when trying to use the phone one-handed. However, a useful mod has appeared that will allow you to wake the screen from the much more easier to reach volume keys.
The process itself looks relatively straight forward although you’ll need to make sure you have root access on your Xperia S. You will have to download one of two files (the first enables to wake the screen via the up volume button, whilst the second allows you to wake the screen via both the up/down volume buttons). The full instructions are here and it is reported working on the .73 firmware.