a) Look beyond the obstacle or item that you are riding over - focus on the next item on trail ahead. This means keeping your head up and looking where you are going.
b) Loosen up. Keep your arms and legs soft to absorb bumps. Move around on the bike - you can move your weight back by sticking your bum out on descents, sit further forwards on the saddle on steep climbs and press your outside pedal hard around corners.
c) Riding with others means there is often someone else who is just a little bit better than you. Follow them and see what they do and what lines they take. By the same token, help those who are looking up to you.