Yeah, I'm a prissy new mountain biker too! I don't particularly like getting dirty (me or my bike), and I hate mud! Perhaps this comes from years of riding on the road? I've fallen enough while mountain biking (and yes, gotten dirty and a bit bloody too) to realize it would not be wise to wear my pretty blue Shebeest shorts or my prettiest pale girly jerseys on single track. Black socks are a good idea I hadn't thought of. Lipstick - yes - just a little.
Fortunately, my mtb is older so a bit beat up, clear-coated aluminum and not nearly as prissy as its rider. If I'd bought a brand-new bike, I probably would have ended up with a prissy pale blue (or eek, even pink!) and been even prissier-than-thou.
Loved your post, Bluetree. Can't wait to hear how the mountain biking goes! You have great friends. 
Emily
Emily
2011 Jamis Dakar XC "Toto" - Selle Italia Ldy Gel Flow
2007 Trek Pilot 5.0 WSD "Gloria" - Selle Italia Diva Gel Flow
2004 Bike Friday Petite Pocket Crusoe - Selle Italia Diva Gel Flow