Do you get much exercise, like walking or running? Walking will definitely help increase your back strength and make it easier to sit up straight and have good posture for meditation, and in life in general. Walking at least 2 miles per day will make a big difference after a week or two.
Thanks, yes, I work out almost daily, including hundreds of crunches. I'm going to practice meditating for short periods of sitting up straight, then try to increase it little by little. That might get me used to a "correct posture". Like Esteban said, anything is better than nothing and meditation, like anything, can be improved over time.