OK, this one is pretty basic. All of the books talk about rest and how to get it. Jams, knee-bars, etc. Here's my favorite.
When climbing on non-overhanging terrain, when I come across a decent foothold, I rest on it. Frequently I can do this "no-hands". What about small holds? Rather than try to do a tiptoe on one hold or stem between two holds (which both tire out my/your feet somewhat), I assume a particular position.
Flattening my body against the wall as much as possible, I get the foot at a height that would be borderline high-step, and get my big toe/ball of my foot on the hold. I then bring my body to rest on that foot by moving over above it such that my pelvic bone rests on my heel. The position of the foot is pointing mostly downward, with the ball and big toe flat on the hold. The heel points upward, with the back of it nearly in contact with my... You get the picture, Be careful not to do this dynamically! The other leg is let hang down below for a smear or a hold, essentially to rest. The knee of the foot being rested upon is bent fully and pointed out away from the body. The inner thigh of that leg is flat against the wall. On vertical walls, some hands may be required. Relax them ALMOST to the point of slipping off, to insure that you aren't; you will get a better rest that way.
On low angle climbs and slabs I lean my chest into the rock, bow my shoulders inward, turn my head to the side, pressing my cheek against the cliff to move my center of mass in towards the wall as much as possible. This allows me to dangle my arms, and allow the friction of my shoulders and chest on the wall to provide any necessary balancing force. This allows the hands and arms a complete rest. The hands *could* be left on holds, with minimal/no force applied, but I prefer to dangle them while resting, and increase blood- flow, while "Shaking out."
This position sounds somewhat contrived, and it is. It is also an excellent rest. The
position (for me at least) moves all of my weight/effort to my skeletal frame and allows a
muscular rest. (almost complete enough for me to fall asleep on low angle rock.) You don't
*have* to be flexible to accomplish this rest position, but if you aren't, you don't get
as comfortable, nor as much rest. Balance in this position can be achieved more easily
with increasing flexibility as well.