I inform them verbally as well as on the invoice and make sure they understand the possible damage should the belt or any related components fail. Any customer that purchases a car without guaranteed mileage or history of a belt renewal with complete kit fitted at that time I strongly advise them to have the work carried out.
Also a sign in the office or workshop viewing area asking "Has your cam belt been replaced????" with some pictures of bent valves below it may help get the message across. It's something you could knock up on the computer. I guess we can only do so much.
I have posted something relating to timing belts in the General Technical Forum.

Liam