Difficulty with Meditation Posture

I am able to meditate when on a bus, or when I'm walking, or sometimes standing; however, no matter how hard I try I seem to avoid formal sitting. I can't help but have the feeling that formal sitting would be most effective, and that I'm avoiding something about a formal posture that is also responsible for growth. I think this is merely effort. What can I do to engage in formal sitting, and avoid always only meditating when it is effortless? Should I: 1) diminish the time of sessions 2) change my meditation type 3) change the consistency of my meditations 4) apply something else? In terms of consistency, I wonder if consistent yet short sessions are better than long and inconsistent ones? Thank you.