Calls to action are everywhere for the readers, each one beckoning louder than the last. So when you craft yours, you want to make your message clear as to what you want the reader to do. Once it is clear to you, you need to make sure it is clear to them. Here are few past posts that will help you create your call to action.
3 Keys to Creating a Call-to-Action - 3 characteristics of any effective call to action
So Many Choices - helps you define the importance of your various calls to action
Up Your Readership - helps you increase your subscribers through calls to action
Ask For Action - helps you craft your message