Long is good, but long and straight is even better. This simple range drill can help you hit more fairways.
First, tee up a ball as you normally would, then place a second tee six inches outside the first, on a 45-degree angle. The idea is to sweep the ball off the first tee and swing over the second one as you accelerate the clubhead through the hitting area. Practice this until you can make the move without difficulty, then remove the second tee.
Making practice swings like this teaches you to fully extend your arms past impact, creating maximum acceleration and keeping you from cutting across the ball (and hitting pulls and slices) or swinging too much from the inside (and hitting pushes and hooks). You'll also find the middle of the clubface more often, a must for straighter drives.