The "momentum is your friend" thing is a lot easier to manage on the downhills - if you're scared you can can be hanging onto the brakes like nobody's business all the way down and just let go briefly for each of the bits you need to roll over smoothly. The technical climbing is the hardest (I reckon) because you have to convince yourself to pedalpedalPEDAL to get over things, and sometimes your foot just unclips reflexively and you curse yourself because here you are stopped on an obstacle you can now see you could totally have gotten over if you'd only just kept pedalling. (Err, not that I have issues with this or anything... )