Standards, standards, standards. I use those published by AVIXA (formerly InfoComm) extensively - DISCAS, projected image contrast ratio, audio coverage uniformity, cable labelling for AV systems to name a few. Being able to demonstrate how compliant (or not!) you are with these standards is very useful when it comes to securing investment for refurbishments. Build them in to your tenders and then you also have a solid quality control that is difficult to argue against. Of course not every single installation can comply fully, in which case they serve as good guidelines so you at least khow close you are.
There are also some great guidelines from AETM which are very useful.
