2 hour blasts: a multitool
More than 2 hours: spare inner tube, mini pump, water, first aid kit (about as useful as a chocolate tea pot), multitool, oh and my phone. All located in my Dueter Race X backpack.
If you crash hard enough and knock yourself out, what use is a phone. I wear a bright coloured DH jersey, so increasing my chances of being spotted in the undergrowth/ditch.
I wear a half face helmet and knee pads too.