I also think the WordPress.org forums are the best solution, as long as you're getting a reasonable amount of support requests.<div>At the moment, I'm getting a few email support requests. I answer them, but I also mention I'd prefer them to use the forums so that everyone can benefit from the discussion.</div>
<div>If something will be useful for many users, I'll post a tutorial on my site.</div><div>Here's an interesting article on supporting free plugins that can probably apply to themes</div><div><a href="http://tommcfarlin.com/wordpress-support-system/">http://tommcfarlin.com/wordpress-support-system/</a></div>