Whenever I am taking a long weekend from work and I have only a 3 or a 4 day work week, I find myself amazed at my productivity. Somehow, in 3 days, I can accomplish the work that I usually get done in 5.
The simple reason is that I have no other choice.
I am going to be traveling or on a plane for the next 48 hours, so I must complete everything before I leave or my business will be stalled during those 48 hours.
As Tim Ferris described in the 4-Hour Work week, this is Parkinson's Law.
Your work will expand to the amount of time you give it.
So if you want to work less, simply start giving yourself constraints.
Constrain yourself to a time window to launch a new project.
Constrain yourself to leaving the office at no later than 5 every day.
Constrain yourself to actually taking a real lunch break everyday.
It's easy to let work consume you, and to spend hours working away.
But if you simply give yourself a time constraint, you will perform the same or better quality of work in less time.
So give yourself constraints. You will perform better under them.