Your and You're
You're
You're is a contraction of you are. It has no other uses. This is a 100% rule. If you cannot expand it to you are in your sentence, then it is wrong. Examples:
The trouble with the rat race is that even if you win, you're still a rat. (Lily Tomlin)
(Expands to you are – correct)
Doing nothing is very hard to do. You never know when you're finished. (Leslie Nielsen)
(Expands to you are – correct)
Please ask Joan to post the parcel when you're in London.
(Expands to you are – correct)
You said that you couldn't believe you're ears.
(Does not expand to you are – should be your)
Your
The word your sits before another word (usually a noun or a pronoun) to show that it belongs to "you" (e.g., your car, your arm), is of "you" (e.g., your picture, your photograph) or is related to "you" (e.g., your uncle). Your is a possessive adjective. (Other possessive adjectives are my, your, his, her, its, our, and their.) Examples:
Our expert will answer your questions about pensions and savings.
(questions belonging to "you")
Sarah doesn't look like your sister.
(related to "you")