In addition to stem height that is discussed, remember to keep your arms "soft". You should not have elbows locked and it should be as if your arms are floating. It is weight that is tiring them. To let your arms float, that means CORE strength. Yoga or, my personal favorite, pilates will give you that strength. I used to have a tired upper back and lower back on biking and now, after 2 years of hard core pilates, particularly in winter, my arms can just float there and I don't get these issues, since my back and abs support the upper body for hours, even if I am in the drops.