I used an interval timer for a long time to develop my shot rhythm. I used an app called Seconds Pro that works on my iPhone. I particularly wanted to develop a consistent hold time, as I have had problems with target panic in the past which usually starts with rushing my shot. All I wanted was a beep to nock an arrow and start my draw, and another beep to release the arrow. A 10 second interval worked well for me, as it allowed me to nock an arrow, draw the bow, and hold for about 2 seconds before the next beep.
After some time, I discovered I could do the same thing with breathing, which makes the whole process more natural and avoids the minor hassle of using the iPhone app. I start by taking in a fairly deep breath just before I draw the bow. As I draw the bow, I exhale completely. At full draw, I inhale slowly and hold my breath. I don't start final aiming until I start holding my breath at full draw. After releasing the arrow, I exhale and breathe again as a part of my followthrough. Of course, I'm looking at the target throughout my draw, so when I start my final aiming I'm aligned with the target and at least pre-aimed, but my attention is mainly on my shot process and not concentrated on the target until I start holding my breath.
I've always heard that focusing on breathing is the best way to clear the mind of extraneous thoughts and concentrate, and I believe this really does work when I make breathing an integral part of my shot process. Fortunately, the timing works out about the same as my interval timer.
A strength building exercise I do is to hold the bow at full draw, both left and right handed, for an extended period of time before releasing the arrow. I used to do this by setting my interval timer on 20 seconds. I have found that three breaths approximately equals this hold time, holding my breath after the third inhale and firing the arrow normally. I do this close to the target and focus totally on form. Of course, it's pretty hard to hit anything after holding for 20 seconds anyway!