If I may have a definite say on this matter of increasing your bow pulling strength...
As far as I am convinced, this is a matter of 3 things... technique, strength and repetition. Once you increase your overall upper body strength, the application of correct drawing technique on heavy bows obviously becomes a much less strained factor. It doesn't even require much exercise to drastically increase your upper body strength, as most people are usually not doing much other "weight" training than shooting their bows.
As already mentioned, 2 basic exercises that can (mostly) be performed at home are pushups and especially pull-ups of all kinds and variations. Pull-ups are king of upper body strength.
Bear in mind, that most people cannot even perform a single pull-up. For some, it may seem impossible to begin with, although, all it requires is for you to get started. Always take it easy to begin with, ensure a proper warm up before you begin, such as arm swings or perhaps even a set of pushups. Proceed to grab the bar with both hands, and then slowly let yourself hang loose with arms fully extended.
From here, imagine it as if you want your elbows to meet behind your back, focusing on proper muscle activation and scapula retraction. You should feel as if the whole motion of the pull-up has its origin amidst your shoulderblades, as this ensures you bringing the strong muscles of your back into play. Do not initiate your pull by contracting your biceps; this will only shorten your range of motion and inhibit deep muscle activation. Pull with your back, not with your arms. If you succed in bringing the proper muscles into play, your arms will follow and lifting your chin above the bar should feel natural and unstrained.
Naturally, we are all only mere mortals, and lifting your own bodyweight might not be as easy as it sounds. If you are unable to perform a single proper pull-up (do not despair, this is the case for the vast majority), simply start at the top position and keep it for as long as you can. When you are at your limit, start lowering yourself as slow as you can. Repeat this motion daily. Within a week or two, you should soon be able to perform a proper pull-up
As with archery, do not get too eager. Exercise only once every other day. To begin with, execute something between 2-4 sets of as many FULL MOTION PULL-UPS you can perform. A full motion pull-up being the previously described. Let yourself hang loose at the bottom of each repetition. This will force you to focus on proper muscle activation. As you get to try these things, you will quickly learn that lifting heavy things suddenly becomes much easier, when you learn to "recruit" the correct muscle groups. If you perform these exercises with serious intent on a regular basis, results will come quickly, usually within a matter of weeks. I guarantee you this.
Pull-ups aside, the mere act of pulling your bow (with or without an arrow) is probably the closest thing you come to an exercise that resembles your draw, and should not be neglected. I do this myself on regular basis, alternating between my right and left hand, for optimal results. I warmly recommend this to anyone who is not already doing this.
Wrapping it all up, the moral of the story is... repeat and repeat with proper technique, and there are no limits to how strong you can become
