Whilst there are no "road rules' when trekking, there are a few conventions that should be followed to keep everyone happy and make for safer trekking. Of course, not everyone subscribes to these... with sometimes dire consequences.
1. Those travelling downhill should give way to those travelling uphill when the track gets narrow. It's hard work going uphill at altitude. Don't spoil someone's momentum by charging down and forcing them to stop!
2. Give way to porters, whichever way they are going. These men and women are earning their living and they are carrying loads way bigger than you'll ever carry. Don't make them wait for you to pick your way up or down the trail. Step aside and let them through.
3. Give way to yaks/mules at all times. Try and stand uphill of them. Don't stand on the edge of the trail. You could get knocked off the side of the mountain by a swinging head, or wide load. Don't try and overtake a yak/mule train. They startle very easily and again, you could be knocked off the mountain, or someone else could get gouged by a horn if the yak veers away from the idiot looming up from behind. Just be patient and wait for the train to pass.