I learned to hit a "straight" or straighter FH by incorporating these points:
- I space the ball away from the body correctly.
- Contact the ball more out in front. Think backing the hitting arm with the shoulder.
- Think slinging or flinging the rackethead.
Hitting a straighter FH is a challenge for most rec players because, IMO, not many are 'brave' to distance themselves far from the ball and swing all out. People are more comfortable staying close to the ball as if they'd have more control. And you need to swing all out in a slinging motion to get power. Double bent FH is easier to get power and comfort (and the limits that go with it) because you'd be doing something like an arm wrestling.