Full disclosure - I work for PR Newswire.
First I'd suggest thinking 'content' rather than 'press release' and apply all the best practice to it that you would to content on your own website, i.e. who is the target audience, what problem are you solving for them, do they value text or multimedia, is the content unique and compelling, and what do you want them to do with it?
Second, use online distribution as part of a strategy to create a broad link profile; too much of any one thing can be bad for you.
Lastly, look for distribution services that publish your content widely and to relevant and authoritative media as this will give you the best spread and likelihood of reaching your target audience. Online PR works and we've had success using it to promote our own content and growing the audience for our blog.