I had the same problem with my 9 year old and here's what I did: I gave him the homework to do at the kitchen table near where I was making dinner. I'd turn around every once in awhile to see if he was done. Before he was done, I'd ask him, "are you finished?" Of course since I was right there and he knew I knew he wasn't finished, he's say "not yet." Then I made a big deal out of it (the Comment part of the CLeaR Method): "Good for you! You're telling me the honest truth!" Then I gave him a label: "I like dealing with an Honest Man!" And then I gave him a reward: "Here Sir, is a big glass of chocolate milk for being a straight shooter!" He just loved this - I did it with him over and over and soon, he'd tell me the truth even when we weren't in the same room and  I had no way of knowing whether or not he was finished.

My 8 year old also had a lying problem. He lied about where he was and what he did so if we asked him, "did you make that mess in the living room?" he'd say "no" even though he did it. If he knew he shouldn't have been doing something, he'd just deny that he did it. So we did two things. First, we stopped trying to trap him by asking if he did something "evil." Instead, if there was a mess in the living room for example and we knew he did it, we'd just tell him to go clean it up. We didn't create a situation where he had to lie. The second thing we did is used the CLeaR Method. If he gave a lying answer, we asked him to think again and tell the truth. Then if he came up with the truth we told him, "Thank you for being honest. You're getting to be quite an honest young man. I think you should stay up 15 minutes longer tonight for that." He saw the benefits and soon was telling us the truth and skipping the lie.