We're not a web design company, but we regularly end up contributing our two cents worth to how a client's site looks. We know our way around SEO, design theory, analytics and so forth, and we're generally nosy parkers, so we often make recommendations. This is doubly true when we're helping clients with their CPC campaigns. It's surprising how many websites we encounter that don't follow best practices with regards to converting site visitors into 'users' (where 'users' might mean buyers, petition signers, subscribers and so forth). It's not rocket science, but it seems like many organizers (or their design agencies) emphasize other, less important design aspects in lieu of optimizing their conversion rates.
When discussing this with clients, I often refer to our Big Dumb Button theory. That is, ensure that you've got a big, obvious button that features your call to action. The Mozilla Foundation offers a good example of the Big Dumb Button theory in action. No fuss, no muss and no ambiguity--just a big green button.
It's also important to have the same call to action linked in the text, because studies have shown that some users will skip the graphical content altogether, and focus on the words.
I was reminded of this topic by an excellent article on Boagworld:
Having an effective call to action is an essential part of any website. A call to action is not just limited to ecommerce sites. Every website should have an objective it wants users to complete whether it is filling in a contact form, signup for a newsletter or volunteering their time.
It has an ugly cover, but I'd definitely recommend Call to Action by Bryan and Jeffrey Eisenberg. It has tons of practical, tactical advice for anybody in charge of an organization's web presence.