From Search Engine Watch
1. Keyword research. Know what your target audience is typing into search engines, and use those keyword phrases in your press release.
2. Keyword placement. Use keyword phrases in your press release headline, in your lead paragraph, and a couple of other places in the press release.
3. Use subheadings with keyword phrases. If you use a single keyword phrase three or four times, you have optimized the press release.
4. Company name. If your company name contains a targeted keyword, take your rough draft and replace the “its” with the company name.
5. 200-400 word count. This is a good range for both search engine visibility and PR.