Getting a lot of visitors to your web site is great, but it doesn't help your company's bottom line unless they actually do something once they get there. A call to action is the doorway that turns eyeballs into engaged customers. You want your calls to action to be the best that they can be, which typically requries regular tweaking and monitoring.
So how can you optimize your calls to action?
First off, if you have only one call to action that is listed on every page of your site, you are probably doing it wrong. Sure, making a sale may be your overall goal, but there are usually multiple steps in the sales cycle. Because of this, there are also several different types of calls to action. Some may ask for the sale, but others may simply provide more information on your business or get visitors to engage by providing contact information.
The key is to have the call to action match the individual page's purpose. Often, each page of the site represents a different step in the sales cycle, so the call to action on that page should be relevant to the content and provide the logical next step for the person reading it.
A call to action will not get much response if people do not even see it.
There is eye tracking software available that you can use to determine where people are looking on your web pages and whether they are seeing your calls to action. You can also utilize Google Analytics' In-Page Analytics feature, which will show you where on each page people are clicking. These tools make it possible for you to do free A/B testing by placing the exact same call to action at different points on the page and seeing which placement produces the most clicks.
This still leaves the question of design, which includes both the way that the call to action looks and the specifics of the offer itself. A highly relevant and visible call to action might still be ineffective if no one wants to respond to it.
Remember that any call to action is asking people to do something in order to receive something of value. It may be as simple as entering an email address in order to download a free eBook. You can increase conversions by wording your offer to either lower the apparent cost (such as the risk of getting spam email in return) or by increasing the apparent value being received.
One quick way to test designs is to have different calls to action at the same location on each page, and then see which page gets the highest conversion rate. If one call to action is highly successful, figure out what makes it different from the rest and incorporate that into all of your designs.