So you're struggling with your ballstriking. Maybe it's your takeaway? Maybe it's your impact position? This drill will help hammer out any issues.
Most lessons involve some sort of laser- or radar-based technology to confirm that your path, angle of attack, swing direction and any number of other measurements are correct. These devices work off a zero reference line; the closer your takeaway is to the "zero line," the straighter your shots will fly. It's invaluable feedback. The problem is, it's not always at your disposal, and it's usually far too expensive for most golfers to own and take to the range at their leisure. So unless you have nonstop access, technology can't be your only guide.
The next time you visit the range, set two alignment sticks on the ground, each pointing at your target. Place one in front of the ball and the other behind it (as I've done here), leaving enough space between the two so that you can hit shots without hitting the sticks. As you blast shots, swing the clubhead along the sticks on both sides of the ball. Keeping the clubhead along these lines is the same as "zeroing out" your swing path on radar, and unless your clubface alignments are out of whack, you're going to hit straight shots. Call it a "poor man's Trackman." It's cheap, but it works!