Your Feet Can Fool You
One of the most common mistakes I see among my pro-am partners is over attention to the alignment of the feet. They take great pains to set their feet properly while neglecting the position of their hips and shoulders. The fact is, it's easy to look and feel square to the ball while in reality being in a markedly closed or open position.
Often, the player initially sets his hips and shoulders just as accurately as his feet, but in the process of waggling and getting comfortable over the ball, he twists his body out of the square position, usually into a more open alignment.
So don't be fooled by your feet. Ask one of your friends to take a critical look at your address position, and if you're out of alignment, tell him to turn your shoulders until you're where you ought to be.
Alternatively, take a club, set it across the front of your shoulders, and see where it points. If it's off, realign yourself until the club points parallel to the line connecting your toes. Chances are, this new alignment will feel uncomfortable, but it will be the beginning of straighter, more consistent shots.