Hi J.,
We have had this issue and didn't handle it well. I facilitate workshops on teaching our values to our children and have some thoughts on honesty. It's an hour workshop but let me give you some bulletpoints and you can contact me for more if you are interested.
Children lie because they don't want to disappoint us or because they are afraid of consequences. Your child, at 8, can understand that lying is wrong.
Would you be willing to switch your approach to rewarding honesty and not punishing lying? Not ignoring but talking about the importance of honesty and then rewarding it every time your child experiences it. It's awesome that you are teaching this now when the stakes are homework and not when she is older and those stakes get dangerously higher!
I have a 15-year old daughter who went through the very same thing at that age (and younger) and the mistake I made was not providing her with a safe environment where she could tell the truth. Of course you are angry when they lie but you have to look at the big picture and see that they need to be able to tell you when they make mistakes (think: call me to drive them home from a party where there has been drinking).
My opinion is that we have to teach what we want to see (proactive) - not assume they are just picking it up through osmosis. Lots of time, we as parents, think our job is to discipline only (reactive).
Here are some quick tips:
1. Make sure you are always honest. Even asking a child to tell Grandma that mommy can't come to phone right now (just and example) or other "white lies" can show that your committment to honesty is conditional.
2. Don't ask questions you already know the answer to. This is HARD but it is setting your child up to lie. Maybe instead of "do you have any homework?" it's "let's look at your homework".
3. Be careful not to label. None of us want to or try to do this but it's frustrating to be lied to again and again. No - "you lied before so how can I believe you now". Take it from someone who learned the hard way.
4. If you think your child is telling an untruth, stop them and remind them how important it is to tell the truth. When they tell the truth, commend them for their honesty even though it was probably really hard.
5. Look for examples on TV or in books of when someone has not told the truth and ask your child to explain what happened. Do the reverse with honesty. "How do you think they felt when they told the truth?" Kids have to EXPERIENCE to learn. Hands on as much as possible.
If punitive consequences aren't working - I say stop them immediately. You are losing authority. All kids have a "currency" and your daughter's is not priveledges. I have one who could be grounded from everything and she would care less. Her currency is words of affirmation. She likes to be respected and told how mature she is etc. etc. This kid could care less for a sticker chart (rewards) either but that may work for your daughter. Telling the truth earns a sticker or check mark and, at the end of a set time, she gets to pick an activity to do just with mom - no little sister.
OK, this is so long but hope you got some insight. Let me know if you have thoughts, questions. I wish you luck and I wish your daughter success.
T.