I have been a mountain bike rider since 2000, I do a lot of trails, singletrack and even downhill. I have some experience with crashes and carrying cameras. My most sincere advice as both, a biker and a photographer is, you can't do both.
Most of the time having delicate equipment in your backpack keeps you from fully enjoying the ride, because you'll always be trying to protect it. So when I say you can't do both I mean you should start your ride deciding whether it will be a mountain biking ride or a photo tour, and plan accordingly.
I assume you also have experience in biking so I have nothing to say there, but if you plan to take your camera with you, my suggestions are:
- Plan your ride so it will be a photo oriented ride. Think of enjoying photography rather than enjoying biking, the bike will be your means of transport, a secondary topic.
- Preferably ride routes you already know and feel really confident. I mean that you master the techniques you need to ride the trial and feel no special or excessive difficulty. Ride difficult trails several times before taking the camera there.
- Given that you know the route, plan the spots where you are going to take the pictures. If you go and try to find every single good shot, it may be more convenient to take a hike rather than a bike ride.
- If you have alternatives to get to the points where you want to take photographs, take the route you feel easier.
- If other riders are going to accompany you, choose people who wont be complaining for you stopping constantly or will be hurrying you all the time. (You won't enjoy either riding nor photographing, and you won't let them enjoy their riding).
- If you are going to photograph the other riders in action, let them know beforehand and be sure they are willing to pose, repeat maneuvers and go back and forth on the photographically interesting parts of the trials.
- Maybe the most important: Modify your riding technique, take it more on the safe side, don't speed as usual, don't try to conquer the usual obstacles, walk down the riskiest parts even if you feel confident. You can't plan a crash, so you can't know if a protective bag will be enough against a specific crash (You may hit a log, a rock, a low branch...). Also, if you don't mind, adjust your bike's suspension to be softer and slower rebound. You'll fell the bike a little sluggish but that will reduce shake and vibration to the camera. Use a standing riding position while crossing rock gardens if your suspension is not soft enough or if you don't have a suspension bike.
- Consider investing in good quick access backpacks (As said in previous answers, your body will absorb vibration). I suggest the ones having waist straps and chest straps, as they can be snug-fit. A loose backpack induces more vibration and makes riding more difficult. Even so, most quick access packs, only allow easy access to the camera, the accessories have regular access methods, so be really patient when changing lens, engaging a speedlite, etc. Be really careful not to forget little pieces of equipment or to carefully zip close EVERY pocket in your pack. (See point 5).
- Plan for the micro weather. Sometimes, mountain biking takes you to places with different temperature and humidity than the general weather, as for example, near a waterfall, a highly forested canyon, windy mountain tops. Prepare to protect the camera in these situations.
- Finally, if you prefer to enjoy the ride, the trail is unknown, are going with people you're not familiar with, have high risk of bad weather, etc, It may be better to take a point and shoot, or an inexpensive camera/lens if you have one. The P&S is esaier to carry and protect with a simple plastic bag or ziplock and can be carried in a small pouch attached to your chest-strap bike specific backpack, so it will be easier to take in and out for that quick pic. This is also advisable if you do a ride as part of the planning stage for a more defined photo session, i.e. to make draft shots.