If you're riding a trail for the first time consider it a recon mission. Take it easy and check everything out. No point hitting something with no knowledge of the take off, distance, speed required or landing condition. This goes doubly in the bike park - always take a few warm-up laps, especially on the jump trails which get reshaped periodically.
To add to the new rider stuff: I once brutally overestimated a new rider's capability and took flak for it for years after. Remember that we've been riding this stuff for years and have likely forgotten all the fears overcome, skills learned, crashes taken and the fact that most of these trails are very familiar to us. I developed a system where I would take someone super new to SFU and go over the basics. How to get behind the seat/riding down steep things, shift forward/standing/climbing, stepping over obstacles/bunnyhopping, the basics of braking, shifting, etc. That way at least I could see first hand what they were capable of. Then I'd take them on a loop where all the challenges came at them sequentially and from a ways off to ensure they'd have time to see an obstacle and have time to prepare for it.
Don't forget/underestimate how terrifying the trails around here can seem to a noob.
For advanced riders: yoga (flexibility and breath management) and weightlifting for power and bracing (in particular squats, deadlifts and kettlebell work).
There's nothing better than an Orangina after cheating death with Digger.