Think about it. If you have at least one client you should be building a strong working relationship and your creating trust. Assuming you always meet deadlines and you're customer service is par none, there is no reason why you shouldn't ABC - "Always Be Closing".
This is how you increase your billable hours.
You don't have to be pushy and all salesmen like about it, but you should always be aware of additional ways to add value and help your clients see that by giving you extra work you are providing adding benefit to them and their business.
Check out my video below to hear more on this topic so you can start billing more hours with your existing clients without having the added stress of having to find new clients.