This was originally my "Holy Trinity," but ever since the fourth member appeared, it became the Holy Quaternity. These four things that you see in the photo below are absolutely crucial to my sanity while hiking. Without any one of them, I'd be utterly lost and unable to continue.
First, there's the white washcloth. This gets folded into quarters and tucked under my backpack's right-shoulder strap, thus reducing compression and staving off strap-related pain for most of my day.
Second, there's the dark-blue handkerchief, which also gets folded up and tucked under a backpack strap-- the left strap this time.
Third, there's the absolutely essential black strap that is acting as a substitute for the chest strap that popped on my very first day of walking. This strap is looped around both shoulder straps and tightened if necessary.
Fourth, the newest member of the group is the large white towel, whose main purpose is to play the role of handkerchief and wipe away any excess sweat that isn't intercepted by the bandana on my head.
All four of these items are indispensable to me. I really would be stymied without them, so they're at the top of my preflight checklist whenever I'm about to step out the door or leave a break site.