I'm often asked for advice on pay-per-click ads (usually Google AdWords) and specifically, for tips on copywriting them. As usual with this stuff, someone else has already done the research and published their answers, and in this case my favourite article is Killer PPC Ads: The Fundamentals on Search Engine Watch, which was published last summer.
This terrific article gives eight AdWords copywriting tips for you to choose from (you're never going to cover all of them in three lines of copy!) and a whole lot more good ideas too. There's a follow-up article too which you might also like.