When you do spend time sitting, sit on the front half of your chair with your shoulders back and down, feet flat on the floor. If you're doing computer work try to get your lower arm parallel to the floor. Visualize a string from the top of your head to the ceiling (that works as a visual while riding also--think of yourself like one of those bumper cars at a carnival! LOL). Just sitting and holding that position with strengthen your core and give some of the right sort of muscle memory for your upper body.

I learned that from playing music actually! LOL Music instructors are as bad as riding instructors, "sit up straight! shoulders back! feet flat on the floor (heels down)! head up! now relax you're too tense!"