You're vs. Your: If Your Not First, Your Last

Kudos to @galensmomma for tweeting an important life lesson:

It's a good idea to have somone proofread your work, no matter how simple.

Where does this truck owner go wrong? He doesn't use the contraction you're as a shorter version of you are.

Quick rules: 

• Use your to show possession. We're laughing our butts off at your grammar blunder.
• Use you're as a contraction for you are. We hope you're taking grammar more seriously.

Correction:

• If you're not first... you're last.