Here's another hard and fast rule I have: when hiking with others, agree on the rules before you hit the trailhead.
By that I mean, discuss the various scenarios that might arise, and agree on what you'll do if each comes up. So, for example, I'm always concerned that my or a companion's judgment will be impaired during the hike (because of altitude sickness or just plain hardheadedness) and that this might lead to dumb decisions about whether to keep going or turn back. So one rule I have w/ my hiking partners is, if one person says "we need to turn back", then -- no argument -- we turn back.
That rule might seem too hard, but the point is that you can avoid lots of problems if you carefully think through all the difficulties that might arise, and are clear on how you'll handle them.
