I have the same problem with my 5 year old. We did the hole time out thing, The main thing is to keep calm, and get down at there level and talk calmly and ask them to stop and think about there answer first before they make a mistake and lie. My 5 year old would STILL lie after I would talk calmly tell them I still love them but lying is wrong. I would take away his nintendo, and tv. Don;t take away activities like sport practice,or healthy stuff like " were not going for a walk after dinner because you lied" you don't want to discourage healthy activity but take away TV and replace it with coloring and or doing puzzle, and keep reminding them lying is wrong. When my 5 year old would tell the truth I would reward him.. Give him one of his treats. I had to start doing this.
Everytime when you know he/she is lying then send them to bed earlier or send them to their room or another thing you could do is ban them from something they like to do, or pla with such as a nintendo DS or something like that, you could also ban them from sweets and sugary foods, all this stuff worked on someone I know
Explain to him the price one pays for lying,and don't ever lie to him,for instance making a promise and not keeping it a lie is a lie,I wish you luck
If you ever know he is lying say I know all about it. You are lying to me! If you don't tell me the truth that is exactly what I know you will be grounded.