Ben,
in the weight training world, we have a term called ladders.
Example: 1 pullup, 10 second break, 2 pullups, 20 second break, 3 pullups, 30 second break, and so on.
The great thing about ladders, is, that it will make you a hell of a lot better at doing one specific thing (like pulling the bow). I've transferred this to archery, even though I've modified it a bit. You may find it interesting.
Pull and hold with right hand for 1 second.
Switching side...
Pull and hold with left hand for 1 second.
Switching side...
Pull and hold with right hand for 2 seconds.
Switching side...
Pull and hold with left hand for 2 seconds.
And so on...
Don't have to pull with your left hand, I just do to even things out. You can also choose to count to 10 or so. The important thing is, that you climb the ladder up until the point where you can feel that it will soon be too much. This is where you start to climb down the ladder again, pulling the bow for 1-2 seconds less each time. Do this regularly, and laugh in the face of fatigue!
I hope that this was useful. If you have any questions, please, do ask.