Ask HN: What activity should I spend most time on?

I am a software engineer turned freelancer, and I started a company some time back with a friend.

I am good at making stuff and my other co-founder is good at design & research.

We have a couple of clients right now that are helping us pay our bills and take care of a couple of employees (excluding us).

Obviously, for increasing the revenues we should focus on increasing the sales and marketing. Right now I am not sure should I focus on just catering our existing clients(which is a risk) or should I spend time on some other activity too?

PS: We provide Web design and development services to our clients.