T.H.
here's what we do in my house...
I have a 7 yr old and a 3 year old and my husband doesn't get home till very late so it's just me and the kids for the most part. When we get home from work, school, daycare, etc. I set a kitchen time for 30 minutes - the kids can play, watch TV, go in the backyard (as long as they are in earshot of the timer)it's free time for them. Then as soon as the timer goes off they know it's homework time.
I then set the timer for 15 minutes and they know they only have to work for 15 minutes. If the homework isn't done, they get a 5 minute break via the timer (they love the timer) and then another 15 minutes of homework with 5 minute breaks until it's done.
This really breaks up the day for them and they know they can do anything for just 15 minutes at a time. The timer adds a little fun to it.
Good Luck!