Break homework time into 15 or 20 minute segments and alternate work time with reward activities.
Rewards can be anything more lighthearted:
Mix it up to keep your child interested. Indoor, Outdoor, active, calm, you know your child best. [Ideas: snack, outdoor chalk drawing or jumping rope for the younger set, shooting basketballs, or game time.] Reward activities should also be done in 15 or 20 minute segments.
For speed and ease, nothing much beats a simple timer set to 10, 15 or 20 minutes. But if you have time, money, or inclination go for something more fun: Try ooze tubes or glitter jars or sand timers instead of being hung up on the actual minutes. They turn over the tube or jar when they start working or playing and stop when the tube or jar is completely settled. They shift activity with each flip of the tube or jar.
Ooze tubes take between 10-15 minutes to empty from one compartment to the other. They cost about $5. They come in a variety of colors. You can also find versions that are the same time but cost more and are larger. These are the favorites of my kiddos. [We also use them for calming down.]
Glitter jars you can make yourself.... maybe even have your child help to get them involved. The time length and costs vary depending on how you make it. You can fin a link to directions on my Pinterest site or google for the directions of your choice, there are many different versions.
Sand timers come in all time lengths and designs.... from My favorite site for finding fun sand timers is officeplayground.com [which also has a wide variety of stress toys].
Note: You can also use the "bite-size" approach to help your child shift between efforts on different subjects, making sure that math, science, history, and grammar all get a turn at the homework energy.
Image credits and thanks to: officeplayground.com, edu-nova.com