Here are several good checkpoints to make hitting long, straight shots surprisingly simple. These go along with proper swing plane discussed here: Minimizing the Likelihood of Back Problems
Some checkpoints are primarily for straighter, more accurate shots. Others are primarily for greater clubhead speed and longer shots. However, they work together.
To start with the simplest adjustments go here first: Simple (and Easy) Swing Adjustments, then come back to this page.
First, the checkpoints for straighter shots:
Grip – your hands should be parallel to each other if opened up from your normal grip position. The back of your leading hand should be perpendicular to the target line.
Clubhead position at waist level on your take-away – the toe of the clubhead should be pointing straight up when at waist level. On the follow-through it should also be pointing straight up when at waist level. Both of those positions are square – not open or closed.
Hand position at the top – Your hands should be above your rear shoulder – the shoulder not under your chin.
Wrists – your lead wrist – left wrist for a right-hander – should be straight. It should be aligned in a straight line with the top of your forearm. This will keep your clubface square to the target. It should be straight until after impact. Wrist cock and uncock is achieved by forearm rotation, not bending the wrist. Check that your lead wrist is straight or slightly bowed at impact.
Clubhead position at waist level on your follow-through – This is a repeat of the tip above. On your follow-through the clubhead should be pointing straight up when at waist level. This position is square – not open or closed.
Now, the checkpoint for longer shots:
Start your downswing with a move of your hips toward the target. They then turn toward the target as they move. Your upper body, arms, and hands are all dragged down toward impact. Your arms and hands don’t become active until at least waist level.
This hip motion reduces the radius of your swing. If this is new, you may need to move slightly closer to the ball at address.
The reason this increases clubhead speed is that your body muscles are much stronger than your arms and hands. You are using the strength of your body and then adding the strength of you arms and hands to the speed of the clubhead at impact.