Salomon has a great video series on "How to Trail Run." Check out this video: https://www.youtube.com/watch?v=YLScRlispoo that focuses specifically on downhill running. There's an uphill running video in the channel as well.
There are so many factors at play in uneven terrain (core strength, foot flexibility, skill, etc), but I think one of the most important factors is confidence! Sometimes I get nervous on technical terrain; however, having a positive mindset helps me push past my fear and bomb down trails as fast as I can!
Hope this helps!