I've been working from home for going on 6 years. I have the same problem. It doesn't help that I also have a young child at home as well. He's only known me to work from home, so every few hours I get forced breaks, which is sometimes very nice to have.

I am with the other commenter that having some sort of activity to transition yourself from work to home is important. Sometimes a jog, or walk is enough. Most days I don't have anything to transition with beyond cooking the family dinner.