Assuming you don't have access to a gym, one thing to try is to obtain a strong theraband. Hold the band with both arms out in front of you, elbows slightly bent, and extend your arms out to your sides. You should feel this in the upper back muscles. If this gets easy, double up the band. Eventually that should provide enough upper back strength to try a pull-up. (You'll also need to do some biceps training, which you could also do with an anchored theraband, or with household objects, if you don't want to obtain dumbbells.)
If you have access to gym equipment, then pull-downs with a lat bar and bent over rowing train most of the muscles you will need.
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.