Try this to ensure your clubface is square at impact. Grip a tennis raquet, (or badminton, squash, etc.) or even a broom and grip it as you would a golf club in your address position. Ensure the face is square to the target line. As you start your backswing, you will see how the face works in trelation to the target line. As you swing down from the top of your backswing, watch the face to see how it closes as it approaches impact. When you get to the impact position, it should be square again.
It could be your swing, but it may also be your grip. If you find that you are unable to get back to square, try this drill:
http://members.shaw.ca/gord962/drills/clubface.htm
It may be a simple adjustment with one (or both hands) to get you back square. I have found with many people that have their left hand closing the club at impact and their right keeping it open, resulting in a straight shot, but after adjusting both hands to square the club are much more consistant ball strikers because their hands aren't working against each other. Others have one square, the other hand is leaving the face open or closed. Very few people have both hands opening or closing the clubface, unless they have a very strong or very weak grip.