The main aim of getting distance with your mid-irons out of fairway bunkers is to catch the ball clean, specifically not taking any sand before you make contact with the ball.
You are trying to make the bottom of the clubface make contact slightly above the bottom of the ball. Grip down the club a fraction, as this shortens the relative length of the shaft, which should raise the clubhead at impact.
Don't lower your feet too far into the sand as this lowers the bottom of the swing below the ball. Play the ball opposite the left heel as the clubhead will be moving upwards at impact.
The sand creates a loose base to swing on, so you will have to stay more centred over the ball and really feel like you are swinging your arms freely and not using your legs too much.
- Take a 7-iron, put your feet together and take your normal posture. Start making balanced swings.
- Focus your attention on keeping your feet quiet and swinging your arms freely.
- Try and clip the ball off the top of the grass, not taking a divot.