If you have a popular keyword product ie make money online, weight loss, holidays etc, you could end up spending lots of money for your cpc and still not get a good position in the search engines.
Before you even put your ad together, you need to decide what keywords would be most effective for getting your website to the top of the search engines. The more specific your keywords that you wish to bid for are, the cheaper they will be and the more successful they will be.
For example, the keyword 'printer cartridge' would be costly as many others are bidding for those very words. Unless you over bid, you will not get high enough up the page to be seen and if you do, you don't know if someone searching under this keyword is a customer or a browser. If you bid on very specific keywords, ie actual phrases that people enter into seach engines, then not only will the bid be much lower (no one else is bidding for such specific phrases) but your prospect is likely to much more targeted.
Although there may be fewer people typing in these very sentences, those that do are highly likely to buy. This makes it much more cost effective way to run a cpc campaign.
Once you have bid for the right keywords, make sure your 1 or 2 line ad is snappy and separates you from your competition in some way. Offering something your competitors are not offering will help set you apart from your competition. This may be a reduction in price, a focus on quality not price, something for free or a unique product. Whatever you do offer, make sure it is appealing to the buyer you have in front of you.
Make sure you test all of the ads that you use. Testing is the real secret to success. Make very small changes to your ad and see if this makes any difference to your sales. Keep making these changes until you come up with the most effective ad overall. Make sure you keep testing and recording as if you don't record, you won't know what works and what doesn't.