Preparation both car and advert.
The car should be as well presented as possible, if a service or MOT or imminent get them done, I had Emblem do an inspection on mine just before I sold it. more evidence that I was hiding nothing. As Matt says the cleaner the car the better, I didn't have a deep detail but I did have it prepped, paint chips, scratches etc all done before I advertised.
The advert should have as many good photos as possible, not blurry images of it stuck on a drive but a good location with all round shots, exterior, interior and engine bay. Any extra special bits should be called out. The words are equally as important, think about what you want to say, I did a brief summary and then a detailed account of Exterior, Interior, Mechanicals and Service History. I also had all the paperwork neatly ready for veiwing including all MOT history, invoices, etc. etc.
Preparation shows you care and most buyers will be impressed by that, mine sold to the first person that saw it.