Have multiple types of work to do throughout the day and access to multiple locations? Try slightly shifting your work environment to stimulate your brain. A recent day of work for me looked like this:

Read newspaper and drink first cup of coffee while sitting outside.
Stand at my desk to do some administrative work and get organized for the day.
Move to outside bar/countertop and a tall chair for second cup of coffee and a couple hours of writing.
Respond to emails in recliner in the living room.
Move back to standing desk to do a couple coaching calls.
Take care of some business reading in hammock on back porch.
Go to office on campus to meet with student and do some academic writing.
Finish off the work day by doing some editing and more emails sitting at the kitchen table.

Your mileage may vary depending on the types of work you have to do but try expanding the various locations you're willing to work in throughout the day. No need to make massive location changes -- even just changing locations within your office or house may be enough to get your mind moving in novel ways. No reason to chain yourself to your desk if you aren't getting the level of productivity you think is possible.

Edit - Great minds think alike? Cal Newport just published an article with a similar idea he calls a "concentration circuit." Check it out!

Photo by me