Form Over Everything
Lifting weights is cool, hitting a PR is also cool, and walking again after a paralyzing accident is super super cool. But all that effort is moot if you muscle through every movement and injure yourself. Listen to those aches and pains, study your exercise videos to find weak points, and most importantly stay humble. You might not need that extra rep, you probably should take today off, and hey maybe even take some time to work on those small boring things that will make the exciting big things work better.
If you find yourself hitching, knee wobbling, squat dancing, or cat backing to complete a deadlift, PLEASE lower the weight and focus your energy on good form. It's all fun and games when you're feeling strong and lifting heavy....until you get hurt. This applies to all lifts and exercises, take videos from all angles, be honest with yourself, figure out the perfect variations for your body, and get to fixing!