Every now and again, I'll adjust my riding position on my bike by trying to lift my bars straight up with my hands. This causes me to reposition my back and hips a little, which puts me back into a better posture.
When I get lazy I start to slouch and hold myself up with my arms, which is bad for my back.
I also like to remind myself to put some weight on my feet - pressing down on the foot pegs with even just a few pounds of force seems to help quite a bit.
