To have a clear "call to action," is not about a protest or support group, but rather laying out a web page so that the visitor does not have to think. Thinking is equal to confusion and confused people do not usually take an action. I will give some examples and guidelines.

When you began to read this post, did you look at this large picture before you began to read the words?
Most people's eyes are naturally drawn to a large contrasting image, so remember this can be good if used properly, but can also create a distraction if used poorly.
PLANNING
Before you begin to make changes to your website, you should first begin with a good plan. Having a good plan can save a lot of time and headaches. If you have and know how to use a graphics program like PhotoShop or PhotoScape, I would recommend doing a mock-up of the page.
LIMIT OPTIONS
Limiting options will make it easier on the visitor. It is good to limit the overall choices to under 3. This even applies to your products you offer. See the home page of my website and you will notice that I only offer 2 search engine ranking programs. Yes there are many variables that I could have offered and given unlimited changes and selections, but simplifying it makes it easy for you and your site visitor and hopefully new customer.
CREATE A SENSE OF URGENCY
When planning call to action, it is a good idea to use wording that creates a sense of urgency. A call to action should clearly tell users what you want them to do.
They should include active words such as:
- Call
- Buy
- Register
- Subscribe
These encourage users to take an action.
To create a sense of urgency and a need to act now, the above words used alongside phrases such as:
- Offer expires May 15th
- For a short time only
- Order now and receive a free gift
- Will help create the urgency you are looking for.
BIG AND CONTRASTING
This concept is very simple. Make the button, you want them to click to do the action you desire, big enough and contrasting enough so they do not have to look for it. White and gray do not contrast, but white and red do and red stands out very clearly from white.
WHITE SPACE AND LOCATION
To make your action button stand out, do not place the item in a location that the person has to hunt for it. Even big buttons get lost if you do not place them in an easy to find location. The center or near center of the page is good, but if the page is long, you may want to include the action button at multiple locations.
White space provides a visual barrier around your large/contrasting button and makes the item easier to find.
POST ACTION EXECUTION
The above basics have been about the page, but the last item to make sure to think about is the post action results. So the visitor clicked the button to register for your newsletter, but is the rest of the process easy, or are they just going to leave the site after taking the action? So the only way to know is to test it. Have you and others you know take a few minutes to try out your process and see if it is smooth or if it needs to be reworked.