There are two areas to consider when promoting your WikiProject - one is in looking for volunteers to help with the project, and the other is letting the general public know when you reach milestones in your project or complete the project.
Seek volunteers to help
Volunteers who would be interested in helping can be found in many places, such as:
Wiki Contributors: Look at who has contributed to existing pages in that topic area in the past and invite them to join in on the project.
Wiki Contributor Newsletter: For larger projects, send an email to email@example.com to submit the idea for inclusion into the Wiki Contributor newsletter. Please submit the suggested wording you would like to see in the newsletter.
General public: Invite those in the community who have special knowledge about research in this area to be a part of the project. One way to find some of them is through bloggers at Geneabloggers (see a list of bloggers by type). Be careful and do not spam them - spam is sending out copies of emails to lots of different people. Do your research and only contact those who you feel really will be interested.
Let the public know about the new resource
There are two times when you should let the public know about your project:
When you reach milestones such as completing major pages
When you complete the project
Consider the following when determining where to let people know about the project:
You can post milestones about your project in all of the same areas above.
The blogging community is also interested in these kinds of updates. As mentioned above with Geneabloggers, they may not have time to help, or be interested in every single milestone, but they will be interested in project completion and some major milestones.