Sitting a the table where you can keep an eye out and never having them sit there doing homework for too long. In other words, if they have 60 or 90 minutes of homework to do, break it up by sneaking  a TV show or somehting else they enjoy in between.

Trust me...I have the classic unfocused teenage kid and he's always been that way. SUPER smart but he's been a real challenge. He is terrible about getting is studies done but is very advanced and has a lot of academic accomplishments under his belt. I personally believe he doesn't fit into the typical learning style used in schools because when  teacher "gets" him, he flourishes.

All of my children have an un-focus issue of some sort. They all get great grades academically though. Focus is an issue here. I find that my 12 yr. old flourishes when he's alone doing homework without any distractions, no tv, no other kids in room. Sometimes he even does his homework in the bathroom with the fan running, I know it might sound weird, but it works. My oldest teenager does his homework with music in his ears the whole time, says that's the only way he can. My last one does his homework in 15 minute spurts then we hit the infamous wall of (No more homework mom I'm tired I don't want to, or whatever else he "feels" at the moment) he has 4 things to do every night (about an hours worth) and it's brutal since I have about 15 minutes of focusing from him. You basically need to quarantine them off and split it up into bite sized pieces of homework. Otherwise it's frustrating for us adults. Oh and my daughter is just fine, (must be just like her mother).lol.

Same with my kids. My boys will focus pretty well, but my daughter is all over the place. What I do is make a after school snack and I sit down at the table with her while she does her home work. Ill either work on my laptop or read, even though she stops me a lot and I don't get much done. 

Ive been able to cut homework time in half just by sitting by her and showing support as opposed to her sitting in her room doing it by herself.