Butt back, chest down! (chin over the stem, same idea whether you stand or sit, no round back, do not hunch, chest pushes towards top tube)
When you are in attack position, check that your chin is over the stem! You should not feel like you have weight on the handlebars. The weight is held by the core and legs, not your hands and arms. Don't round the back but engage lats. Chest pushes towards top tube
Elbows out is the "side effect" of good body position that happens by itself as you assume a good body position, it is not a goal in itself
Here is a link to this (last video on the list)
http://www.pinkbike.com/news/Improve-Your-Body-Position-100-in-30-Days-Blueprint-2013.html
