I also have a "mad money" account for extra stuff. It works great! No feeling guilty about buying countless bags of worms or new rods. I think the important thing is to keep the trust. As someone once said, "where there's no trust, there's no love".
My grandfather has a different approach that he has always used. He pays cash for everything and then puts the item in "hiding" for a bit. Then when he does get it out and is interrogated, he says truthfully "oh, I've had that thing for a while". Once he didn't want to wait to play with one of his new video editing gadgets, so he actually scratched it and spray painted it like one of his old ones to confuse granny. He brags about that and other creative tactics, but what's funny is that Grandma knows he does it and just plays right along with him laughing behind his back. Good woman, they sure don't make em like that anymore!