Hands down, the #1 mistake that beginner, intermediate, and advanced players make is this:
Playing with lips too tense, and too shallow on the harmonica.
“The more relaxed you are, the smoother and faster you’ll run” Usain Bolt once said. And, you know what? The more relaxed your mouth position is the smoother and faster you’ll be able to play harp.
If you don’t cultivate a Deep Relaxed Mouth Position on the harmonica, you will never play with that beautiful bluesy tone that makes us fall in love with the harmonica again, every time we hear it. And if you were to poll every great professional harmonica player and teacher, I guarantee you that they’d agree with me.
The best time to learn the Deep Relaxed Mouth Position is now. And if you’re a total beginner, well, lucky you, that’s perfect timing, my friend. You can get off the starting block ahead of the pack!
To see what it actually looks like you can check out my lesson on it.
Here’s to relaxation! 
Cheers,
Luke