Sounds basic doesn't it. But you would be surprised to learn that many freelancers don't do this.
They feel that they have a great relationship with their clients and when they are needed they will be called. But that's not always the case. Your client might have a few freelancers on hand. If you catch them at the right moment, an assignment that might be added to another freelancer's project could be split up and part of it can go to you.
This is an easy way to increase your billing.... because your clients know and trust you and have no problem giving you the assignment... BUT THEY WERE NOT THINKING OF YOU.
So bottom line: Call your clients from time to time, tell them flat out that you are slow and was wondering if they had any assignments they could throw your way. You only have to get one YES and you are on your way to being more successful as a freelancer.