Its a case of "whats there in it for me. It depends on the context that you have set up before the user reaches the comment section.
Motivation is an emotional response out of
- The content,
- The cheese at the other end and/or
- Sudden discoveries (really useful information/controversial information or comment that forces you to react/comment in this case)
Like on UX.SE I am motivated to see the new questions, issues faced by UX designers, and the responses make so much of learning, its absolutely priceless. Hence I comment, to help out there, to have my say and if I get voted up, it gives me satisfaction.
Similarly, in your case, based on types of users consuming the information from your website, you can build up strategy to get comments.
I believe that mere choice of words are not sufficient to motivate users to comment. Real motivation is when you answer your users' question "whats there in it for me".
A recognition system can really motivate your users to comment more and more. A blog is read only once but it lives because of comments, as users jump in with their views.
Consider Developing a motivating system. Feature the user who commented first on home page. He will be thrilled to see and others will get motivated instantly. Continue with the same system and keep featuring them, under some heading that recognizes their contribution. Like Active Users for example.
Though, to answer your question, it should be simple enough like Add Comment or Have your say etc. But Do you really find them motivating enough to comment? Its the backbone that motivates and not the words.