From Google’s Adwords Editorial Guidelines you find out that The most effective advertising communicates a clear message to a targeted audience. A common sense advice, but not so easy to comply with when all you have is a tiny ad space with less than 95 characters to convey your message.
Google has clear editorial guidelines and clear content policies. But that doesn’t really tell you how to write an effective ad.
There are two reasons why companies employ PPC in their online marketing strategies: to raise brand awareness and to sell products or services. In both cases the PPC ad should act like a call to action that will determine the Web surfer to click on it.
Before starting your PPC campaign take a look at the competitor sites. These appear under the sponsored results for the keywords you target. Then write an ad that will differentiate you from them.
Most of the surfers will read the title of your ad and ignore the rest. So that’s where you should focus your message. It’s hard, because you have just 25 characters, but it is not impossible. Avoid superlatives (terms like best, top quality, number 1, etc) and use relevant keywords instead. By doing this you place the search terms that matter for the users in first line of your ad. Sometimes placing prices in the title, especially when they are really low, will attract clients looking for bargains. (This is, statistically, one of the most successful PPC approaches).
To make a long story short: give your customers a reason to buy from you. That means that with the best ad but a poor page copy, your efforts will lead nowhere. The landing page should have an effective copy that will determine the visitor either to buy your product or to bookmark your page for further reference.