DS2 is similar. looks you in the eyes and lies with a staright face. He'll cry and scream if you call him on it.

*following line may be distressing for some*
"Tell me true and I wont tell you off, lie to me and you'll get a smack" when they are little if they tell the truth they get a kiss

DS2 is 5, so he gets 3 chances to be true. Sometimes he changes his story and sometimes he doesn't. But the policy is there for everybody and its a constant.
I don't think you can stop them from lying, but they can know what will happen if they do.

We also talk about trust and I tell them if they lie then I cant help them, if they tell me the truth then I will always try to help.