It is important to always keep the goal of the Call To Action in mind when you begin creating it.
This will influence the design and copy as you always want to keep it a simple as possible based on the action you're trying to get users to take.
The following are key factors to think of when creating your CTA
Button color that contrasts the rest of the page can bring attention and increase optimization on your action.
Use simply action language, and when possible use terms that are specific to your visitor rather than general terms. For example "ours, "yours", "mine" "my".
Use offers that suggest a customer is missing out if the action is not taken. This has a physiological effect that they will be losing something if they don't do it which has shown to be much more effective.
Testing the size of your Call To Action can change the conversion rate, having the button be too large or too small may effect this. Try testing this as this is one component that can really vary.
Where your Call To Action(CTA) is placed can have an effect on your conversion rate. Adding it to the top of the page, or above fold is highly effective. However if you have content on your page placing it at the bottom can be a way to try and keep readers on the page.
The above are some common components to think of when using Call To Actions. As there's not certain way to determine the best optimized method try doing A/B tests for where you place your CTA.