From an Article in Strings Magazine:
By Judith Glyde
Projecting your sound is not simply a matter of playing louder; it’s often a question of improving your articulation. Both the left hand and right hand must do their parts in this quest for greater sound projection. The steps below will help you be heard on a passage that must project, either in chamber music (especially difficult for the cello or the inner parts in a chamber ensemble) or in solo repertoire with piano. These techniques should be used for fast as well as slow passages.
1. The left hand must always be articulate. In upward passages, tap the finger down. The motion should come from the knuckle joint—be fast and strong (like a piano hammer striking the piano string). The two photos illustrate the “before and after” of the motion—the complete action of the finger “striking” the fingerboard.
2. “Click” the finger off in downward passages. Make sure, when clicking off, that the lower finger is down before the action takes place. The two photos show the clicking off motion. Keep in mind that the hand must not move, just the finger itself.
3. The bow should be placed close to the bridge, weight should be added from the back and the arm, and the bow pulled slowly. It also helps, when playing close to bridge, to add vibrato.