To get this right, answer a simple question:
Does it belong to you?
If something belongs to you – whether it is tangible or not – you don't need an apostophe to prove it. It's yours, not separated or divided.
It is your car, your house, your shoes, your intention, your decision.
On the other hand, if what you are actually saying is 'you are', then you need the apostrophe to show that something is missing.
You're about to embark on a wonderful adventure.
So your reminder is:
If it belongs to you, keep it together.