This depends heavily on geography and individual trail systems. For example, in US National Forests, cyclists yield to hikers and equestrians, hikers only yield to equestrians, and uphill traffic has the right of way (generally, see note). In MTB-specific trail systems, trails will typically be one-way, therefore some who happens to be climbing a descending trail should be alert and ready to get out of the way of descending cyclists.
But to generalize:
Ride within your abilities: this means your ability to complete your planned loop, descending and climbing technical terrain safely, etc.
Be self-sufficient, even on a group ride: If you're in the back and get a flat, your buddy's tube won't help you, since they're now out of ear shot and you are both out of cell phone range.
Bring enough food and water for the ride: self-evident.
Don't ride trails in damaging conditions: this varies by climate, local convention, and geology.
Some trails handle moisture very well. Others form damaging ruts that lead to more moisture retention and vicious cycle of damage. Trail without canopy cover will be susceptible to freeze/thaw. During the thaw cycle, the moisture cannot drain vertically past the lower frozen layer, and horrible damage ensues. This applies to all trail users, not just bikes.
Smile, say hello. A little bit of human connection goes a long way in reducing user conflicts.
Give back: if you're able, join the local trail association as a member and volunteer during a maintenance day. In less, um official trail systems, there may be a "no dig, no ride" ethos. Respect the hard work the trail builders put in. And recognize that a simple, featureless path through the woods may have required years or even decades of tireless and thankless advocacy work.
Report problems and conditions for others: If the local system is on something like Trailforks, report damaging conditions, fallen trees, etc there or directly to the local trail steward. If a tree is down, take a geolocated picture with your bike leaned against it. The tree's diameter relative to the wheel will help the sawyer know how big a bar is necessary to clear it. I'm responsible for several systems near me, but I can't ride them all the time. If people didn't tell me what was needed, planning volunteer days would be shot in the dark
If you can't ride it, walk it: Just because a feature is challenging and slows your progress, you don't have the right to dumb down the trail for the people who are aspiring to clearing the feature or those who already can. Don't sacrifice the experience of everyone else because a certain section of trail wasn't to your taste or above your abilities.
Follow local user conventions: Don't ride trails where bikes are prohibited. Yield to other users who have the right of way.
Pull off the trail when you stop, and stop where people can see you. If there's an entrance to a technical feature that you want to scope before riding, stop well before it, move off of the trail, and be prepared to get out of the way of people carrying momentum through a tricky section. A steep rock garden is a terrible place to have to execute an emergency stop. You don't want to be responsible for someone else's crash as they try to avoid hitting you
That's all I can think of for now.
Note: Downhill traffic yielding to uphill traffic is getting a little long in the tooth, IMHO. That convention was creating when bikes were exponentially less capable than they are now. Climbing was a chore, but descents were more something to be survived than unadulterated fun. A climbing cyclist with decent hearing (and no ear buds) can almost certainly become aware of the other cyclist's presence first. If I'm climbing a two-way trail that I know is a fun descent, I'm typically off of the trail well before the descender even sees me.
