Looks like I'm in the exact same position as you.
We already have stable products and happy paying customets, so I try to limit coding to the minimum necessary - it really helps that I'm not the only one working on the products.
For customer support, you can't really put a limit on that since when it needs to be done, it needs to be done. I do try to write documentation on faqs and common tasks, and that helps a lot for new customers.
What time is left I focus on marketing and sales, but only after making sure that getting a new customer is a smooth and easy experience for all involved.
I'll love to hear about other people's experience on the matter, since it's not easy and there are many different ways to make it work.