Regarding the legal requirements you can get a good idea of what is required for routine logging from the Data Protection site under simple systems at ico.gov.uk.
In most cases CCTV footage can be presented as supporting evidence. "Prosecution" level (CCTV on its own) evidence is much harder to get accepted. You should ensure that your maintenance programme includes wiping the housing windows or the lens if you do not have housings for your cameras. This applies particularly to outside cameras.
Use cleaning materials that won't smear or leave fluff on the glass. This should be done once every few months in general, but could require more frequent maintenance if they are in exposed places eg low down at an entrance barrier. - Cameras can only record what they see and you don't want them to be looking through a fog.
Regualar operational checks are a good thing and also make sure you include a check that operater knows how to backup and verify evidence. Many users of digital systems panick when thay have to back up evidence because they don't do it regularly and are possibly under stress when the actual need arises.
