By the way: A marker of something you need to point out - "By the way, do you know your dress has come back from the cleaners." Mind you: Expresses an opposite point of view. "She is very tidy. Mind you, she can be very messy at times."

"By the way" is when you forget to add a detail. Example, A-"You have a meeting at 3pm, so do not be late" B-"I will not be late" A-" By the way, John will be going with you" B-"ok" "Mind you" is a saying you tell someone to remember a detail they already know. Example, A-"Mom I am going to the park with some friends" B-"Mind you, you have school in the morning so don't say out to late" B-"ok"

Karen is right. You should say "mind you" if you are about to add a detail that contradicts something else you have said. You can also use it to introduce a warning: "In the morning you'll need to feed the dog. Mind you, he doesn't like strangers, so don't try to pet him." "By the way" can be used for pointing something out or adding a detail (as Karen and George have said), and can also be used for making a comment that is off-topic, or introducing a remark or a question that changes the direction of the conversation. "I'm glad to hear that you're enjoying your new house. By the way, have you started looking for a job yet?"