In the case of your specific proposal, I would argue that it is fully covered by StackOverflow and therefore there is no reason to have a separate site. (see my answer to your other question.)
That said, in my experience, promoting an Area 51 proposal on StackOverflow has to be done carefully. You do not want to be perceived as spamming an existing site, especially when what you are trying to do is siphon users to your proposed new site.
Based on the experience of the proponents of the Mathematica.SE site (now in beta), there are many other ways to promote your proposal.
- Post to Facebook/LinkedIn/Reddit.
- Add a link to your profile on StackOverflow.
- Contact relevant bloggers and ask them to post something about your
You have already created a Twitter account - that's a good idea. But you will need to build up a following, e.g. by tweeting interesting SO posts and other information related to your proposal.