Normal setup: (left) This stance can lead to mis-hit shots. Adjusted setup: (right) Turning your toes in creates a quiet base.
One of the biggest difficulties golfers have in a fairway bunker is keeping the lower body stable. If it's a full-swing shot, it's very easy to lose your footing, leading to mis-hit shots. Contacting the sand too far behind the ball is frequently the result. I can't stress enough that you need to be anchored in the sand.
To do this, dig your feet into the sand, then turn your toes inward as you take your stance. This secures your lower body in the bunker. Your weight should feel as if it's on the inside edges of your feet, and when you swing, keep your weight in that same position. Unlike a normal full-swing shot, there's not a lot of weight transfer. This swing is propelled more by the arms and shoulders, and the toes-in position provides the stability needed for a solid shot.
One more important thing to remember for a solid fairway-bunker shot: You should grip down on the club slightly to compensate for the amount you dig your feet into the sand. This helps you strike the ball first.