Here are 3 drills that will promote good form running relating to your posture and mid-foot strike. By repeating the drills over and over again you are creating muscle memory so good form running can become more natural for you.
HOP AND STOP - Bounce on one leg with body and hips slightly leaning forward. Hop forward and backward over a line. Try and stop on demand. If you can keep your balance you are landing with the right mid-foot strike. If you are too far back, you will fall backward, if too forward, you will fall forward.
Practice with 3 or 4 sets each lasting around 30 - 60 seconds.
DONKEY KICKS - Standing on one leg, get into proper posture by raising your hands over your head. Lean slightly forward making sure your hips move forward with your body and move (kick) the other leg back and forward behind you. This promotes good running posture and lengthening your stride behind you.
Practice with 3 sets each doing 15 kicks per leg.
Here's a great demo. Donkey Kicks
KNEE RAISE HOPS - Skip forward while doing high knees. In a sense you are Exaggerating good form by raising knees, landing on mid-foot and swinging arms. Worry more about getting height on your skip rather than how fast you get to your finish line. Pay attention to how your foot is landing underneath you and with the ball of your foot hitting first and then to your general mid-foot area. It's also good practice for keeping your hips underneath you.
Practice with 3 sets of 25 or 50 yard strides.