sounds daft but one thing i do is stick half a tank of fuel in. £15 - 20 fuel, but the look on people faces when they see they are getting half a tank of juice for free.

Every car I have ever brought has been on the red, and to me it says cheap skate. The one car I bought that had half a tank in really made an impression on me, it wasn't until after the sale that they guy mentioned the fuel gauge read wrong and the needle stuck on half way so it was actually on empty