Full disclosure: we are a firm that helps with this.
Of course this isn't an easy, one-size-fits-all answer, but it typically breaks down into 2 components:
Your site needs the right 'technical' SEO architecture - links, tags, etc
The 3 to 5 pieces of content you produce a week need to be high quality content
We specifically help with #2. Typically 80% of the content people publish fails to drive traffic at all. We can tell you how your content will perform before you write it and make tightly related suggestions so the time and effort you put into creating your content will have the impact you seek. Not only will this drive much more traffic, but it will also help your overall rankings.
We typically drive 2 to 4x increases in traffic and related performance metrics (conversions, etc).
Here are a couple recent articles on us:
I'd be happy to take you through what we do, and also recommend some SEO consultants once I better understand your exact situation. I think you'll find we have big impact.
Please let me know a time that works for you this week or next.