Some to avoid too, perhaps?
Generally I am now avoiding sites with few stalls - just-in-case busy. More chance of getting an unpaired stall at a larger site, let alone being blocked because it is full.
Time from turn-off main road, to return back onto it, can be a consideration too.
Also the ability to get access to motorway services from a (restricted) access road, such as Woodall. Last time I was at Newport Pagnell (Southbound only) the barrier was down (I was travelling South anyway)