In reply to bouldery bits:
Sounds like the most obvious advice possible but practice. Just as you'd do hill reps for the ups, you can start working the downhills too in training. Getting used to letting go, changing lines, recovering from the minor mishaps and getting it hard wired that you can go faster than you thought. Plant your feet and try to brake less - easier said than done
That said I'm a worse descender than I used to be after a string of bad ankle sprains - I can keep with people around my pace now I just tend not to make up places.
Watch the weight of the pack, it'll give you extra momentum if you're not used to it.