Oli Dale has put together a nice post outlining methods he's found to be effective for marketing his WordPress themes and plugins. I wrote tips for marketing WordPress products last year, but my list was more focused on what I want as a user / buyer. Oli focuses on what's working for him as a seller.
Oli runs the WP Lift blog as well as his own theme shop, Theme Furnace, and his number one bit of advice for growing either is patience:
Building a blog is one of the slowest types of sites I have done – it literally takes years of constant work to see results and building a good product can also take a number of years to build to the level you want. You can take short cuts by reinvesting profits back into advertising and marketing but I would still prepare yourself to be in it for the long haul. I am entering my third year at ThemeFurnace now and I am still short of the targets I set myself when I started it – I am seeing slow, steady growth though which is what keeps me motivated.