Tension must not be allowed to build up in any single part of your body. Breathing is one of the best things to do in order to prevent the build-up of tension. Always anticipate the next note. This will serve you well because anticipation is part of music itself. Whenever two joints meet, they should form a curve, not an angle.
Angles block the line of power.
A good exercise to help with maintaining a sense of pulse is to alternately lift each foot off the floor. While keeping your forearms parallel to the floor, roll your elbows back and then over your shoulders, clenching the whole time. Release the tension and your arms will be in the proper position, not too low and not too high.