The number of non-orthopedic surgeons in this thread claiming knee braces are ineffective and actually worse for you is somewhat troubling - I think that mentality is pretty much equivalent to those that argue helmets make you more likely to hurt yourself, but anyhoo...
In regard to knee pads, I wear a 661 Race Lite hard plastic knee and shin guard when I am riding park and trees.  I don't notice them riding or walking (I hiked at Silverton while wearing them), you can't see them under baggy pants and they have prevented several impact injuries from rails and trees.  Impact shorts and a padded jacket (several companies make them, Crash Pads out of Oregon for one) are also helpful.
The padded jacket also makes my skinny ass look pretty buff.