I concur with the idea of positioning your arm on the guitar, not planting, to where you can use your forearm as a guide. It is a greta starting point and i think as you get better at strumming you will eventually get away from relying on it ALL THE TIME and learn to fluctuate positions for strumming, fills, and leads.
I am not as seasoned as most of the guitar gurus here on the sight and i am not far removed from having a similar question as the original poster's.
this is my first vidoe share here on the forum and believe you me i am not ecstatic about exposing my weaknesses
this is how i play the intro to UJB, the strumming pattern is down/ downup/ down down/ down up...
I do get a little lazy and tend to rest my arm on the geetar at times, but i found this position to be the best for me to keep time and be flexible with alternating strumming patterns.
http://youtu.be/w_VN1dBKang