Yield the right of way properly
Commonly, trail rules dictate that bikers yield to people on foot (hikers and runners), but everybody (runners, hikers, and bikers) yield to horses. Yielding means pulling off to the side to make room for the person or animal to pass. Of course, many trail users don’t know the rules, or simply might not care, but the more people treat each other with respect and kindness, the more enjoyable everyone’s trail experience will be.
Always maintain awareness of your surroundings
Speaking of trail experience, it’s difficult to yield properly or treat others politely if you don’t even notice them. If your headphones are blasting and you’re hyper-focused on the trail and nothing else, you might not notice, say, a person who’s been tailing you and would really like to pass. By keeping your head up and running with either one earbud or none, you’re more likely to be aware of other trail users and not block their way (not to mention potential wildlife encounters).
Pass other trail users politely
Maybe you’re actually the one who’s been tailing someone slower on a singletrack trail. How do you signal that you’d like to pass? Sure, you could shout, “on your left,” but often all it takes is a well-timed cough or sniff to alert someone to your presence. If not, a polite, “excuse me, mind if I sneak around you?” usually comes across a bit more considerate.