Hi,
Hey, good news, your 7 year old and my 7 year old are apparently long lost twins! My daughter even tried putting a dab of toothpaste in her mouth so her breath would smell like she'd brushed when she hadn't. (Like the fuzzy yellow incisors wouldn't clue me in.) Here's the only tactic I've found that helps significantly:
I don't punish for the infractions as much as I punish for the lie. It's kind of a liar's amnesty (we call it "truth-telling amnesty") - tell me the truth and you get off easier. You even get a cuddle and a handshake for being courageous enough to tell me the truth when you felt nervous about doing it. But lie to me? Watch out.
I gave my daughter a starting phrase to help her get started. "Oh wait, that was a mistake, what I meant to say was..." So she could say, "Oh, actually, that was a mistake... what I MEANT to say was, 'No, I didn't brush my teeth, instead I stole your entire pack of gum and have hidden the chewed up wad under my pillow for tomorrow.'" And while she'll get a consequence, it will be logical and lighter than if she lied-- and she'll also get a cuddle, a handshake and my sincere praise for telling me the whole truth.
Doing this gave her a chance to think better of her reflexive lie and then to replace it with an honest answer. She seemed to like that freedom and before long telling the truth became her new reflexive action.
My fear in starting this was that I'd be doing more damage by allowing my daughter to 'get away' with things. But looking back, I wasn't allowing her to 'get away with it" -- I was retraining her to handle her mistakes more wisely. We all make them, so we may as well learn how to handle the clean-up. I even imagined ridiculous scenarios where my other kids start misbehaving then confessing just to get attention... nothing of the kind has ever happened though.
Literally within two weeks of starting "truth-telling amnesty" she began to straighten out. Things are much better now and (knock wood) I feel like I can trust her to tell me the truth 98% of the time.
Best of luck! :-)