Hi, I've just update (in my opinion) a great site for developers and designers, but I don't know which is best way to promote it: forum posts, blog posts, directory submission or there's better way ?
All the above. I mean that you have mentioned plus if you can get more backlinks to your site so you can get more generic traffic from the SERPs. just my 2 cents Shelly
Contact the local high schools and colleges. Ask the instructors about talking to the students about real world development. From there maybe hand out some business cards. Maybe invite some of the students to join your site. Contact a local college newspaper and talk to them about running an article about your site and your looking for comments from developers.
Talk to similar sites that have the same focus like Developer Resource about cross promotion. Also participate in Network Business of Software its a social site for software developers.