There has been a lot written on the subject of "calls to action" and I've included some links below that you may want to check out. Many of the keys to successful calls include common sense elements like:
- Make it visible by paying attention to placement
- Use good contrast and color choices
- Wording is important (see last link below)
It may even be necessary to use a "secondary" call to action. Since most of us are reluctant to give up our information easily, a secondary call to action can help get us moving in the right direction. The little yes before the big YES, so to speak.
You will often see Calls to Action that try to instill a sense of urgency, as in, "Hurry, before it's too late!" and so on. I suppose, depending on what your product or service is, this can be valuable, but generally speaking, I think most people are too savvy for this kind of approach. Instead, give something of value so that people will feel it was a fair trade. After all, value based transactions can be sustained over time, whereas the urgent or opportunistic approach is more likely to be a one time only connection.
There's more to this discussion and I'll be happy to continue it with you if you'll (warning: call to action ahead) drop me a line.