If you haven't already, make sure to check out What Blogs + Websites Have Provided the Most Help for Your Startup? and What books do you recommend?, as both of those topics will give you a good starting point on where to look for information.
But I believe your best information will come from a good business coach: someone who knows enough about sales/marketing to get you talking to the right sales and marketing people. Out of all of the professionals I've worked with, sales/marketing folks were the most challenging to pick. I suspsect a lot has to do with the difference in each industry (marketing software vs. marketing widgets)
There is a good discussion on How do you find good professional help?, and here's my two-cents on the matter:
I always ask around - whether it's family, friends, or even the business next door. I also go to networking events to try to find contacts.
Once I find a name or two, then I'll schedule an appointment and, if we seem to click, I'll open up with a small project. Maybe a contract review. Or my personal tax returns. Or a logo that's already been designed.
It's simply a trial to show how well they work or don't work. And if they don't, then I repeat the process. Usually, the professional will do an acceptable job (otherwise, I won't pay) so, at the very least, you're only out time.