Work in 60 to 90 minute intervals. Your brain uses up more glucose than any other bodily activity. Typically you will have spent most of it after 60-90 minutes. (That’s why you feel so burned out after super long meetings.) So take a break: Get up, go for a walk, have a snack, do something completely different to recharge. And yes, that means you need an extra hour for breaks, not including lunch, so if you’re required to get eight hours of work done each day, plan to be there for 9.5-10 hours.
I don't normally like to link to these list type of posts since they mostly seem to offer mostly obvious tips. In this case, though, staying focused and productive is easily the most difficult part of being a freelancer, and this list offers some good advice for doing just that.