LeadvilleFan, I'd suggest yoga for cross training. Balance is crucial. Work on core, too. Low body mass is an advantage in trail running with all the shifts in momentum, so don't bulk up . But nothing trains trails like running them.
Also consider getting OFF the trails. Be selective, but there are great benefits to strength, agility and balance in running where 'no foot has gone before'. The need to shorten, lengthen or change direction mid-stride works a treat on your eyesight, reflexes and core. Not to mention the buzz at the end of a steep downhill!