It's so hard to say without seeing your site, but if you haven't come back from Mayday, the problem is probably content.
Give users content they want. Poll your users as they land to find out what that is.
Subtle things can make a difference: ease of navigation, ease of skimming (or if it's multimedia, ease of viewing/listening).
Get somebody's grandma to look at the site and see what she has to say about it. Or somebody in your market who's not very web savvy. Ask them to point out ten bad things about the site. (People you know will always point out the positive unless you specifically ask for the negative.)
And make sure you're targeting the right keywords for your content and not keywords that don't offer value to the users that search with them.