In literal terms, be your own boss in every sense of the word.
When I find that motivation for client projects is slipping I fall into "boss" mode. That is, I set a schedule for work and adhere to it as if I had a boss. It doesn't have to be a strict 9 to 5 schedule, but a block of time -- 4, 6 or 8 hrs -- every day where I will not touch anything other than client projects.
I don't "cheat" and venture off. No random social network/forum/SE surfing during this time. No personal phone calls or text messages. No unnecessary breaks. I treat my office as if I didn't own it and someone else was responsible for my paychecks. This helps me ensure I stay disciplined and get work completed as I need to.
It can sometimes be very difficult to spend time doing things you would rather not be doing at the time. But if you had a 9-5 job, that's exactly what you would be doing most days. There are times where I must simply force myself to buckle-down and get things done. This is especially true if you're own lack of motivation is financially becoming an issue. Remember, slack off too much and you'll be looking for a new job which will probably pay less, if not new clients in a desperate fashion. If that's not motivation for you, it should be.
I also find that after the first 30 minutes to an hour, the motivation is no longer an issue for that particular day, sometimes more. Much like any task in life you really don't feel like doing.... once you start it, it becomes far less daunting that staring at it from across the room.
Here's a related question at GraphicDesign.StackExchange.com:
How to Get motivated When Drained