This may be overkill, but I gained a great deal of insight into my own posture from The Thinking Body by Mabel E. Todd. The basic idea is that the body resists gravity in the same manner that a building does: by distributing the weight along lines of compression and suspension. For the most part the front of your body is suspended from the frame of the skeleton which channels the weight through compression down toward the center of the earth. Thus the bones of the skeleton ought to be balanced to provide this support. Any imbalance is compensated by muscular activity which leads to fatigue. So to fix tension in your wrists,
Start with the feet. Seated or standing, your feet should be solidly on the floor. Heel, ball, toe.
Then, your knees should be slightly forward so the weight from the body is on the arch, balancing between the heel and the toe.
Then, your hips should be "turned-out" like a ballet dancer, then helps to curve the lumbar forward.
Then the back should be straight (seriously, stop slouching).
Then the shoulders should be relaxed and pulled slightly backward (heckles: down).
The test of all the above is whether you can sit for 60 seconds completely still, without needing to move or feeling fatigued (The standing version is actually a Yoga Asana (posture) whose name I forget). Now your weight is evenly balanced down to the ground. When you raise your arms, your upper back should withdraw slightly to remain balanced. Since the arms are suspended from the shoulders, the shoulders pass that additional weight either upon the compression lines through the bones down the spine or upon the muscles of the back, which leads to fatigue.
It might be easier to learn standing. If all the weight of the body is evenly distributed and balanced, there should be no tense muscles at all. You should be able to stand, balanced, with no effort at all. When standing, for the upper body, it should feel as though you are sitting on your hips. And the hips are sitting on the legs. So it is much like the sensation of sitting against a low edge jutting out of a wall.
One additional Yogi trick is to pay attention to the muscles of the abdomen as well. These muscles can be lightly tensed to actually provide some compressive support on the front of the body (so you back doesn't have to move as far).
I know I haven't mentioned the wrists, but if the above is ok, tension in the wrists really should disappear. The only reason for them to tense is if the angle from elbow is too low, which means your back is too far forward, which means you're probably having backpain that you've already grown accustomed to. This too shall pass. :)
The sensation of stability ought to contribute to the feeling of control.