It's important to remember to
always stretch your arms/hands before and after playing a map, and to take breaks if you want to avoid future problems. More information in this useful thread about hand & wrist exercises :
community/forums/topics/649565?n=1You essentially improve your stamina by playing longer/stamina based maps without pausing. We can count there jumpstream, handstream and chordjacks easily found in the beatmap listing. The more you play these types of maps, the more you will get results basically. And of course, you start to play easy maps first to get your hands used to it, then increase the SR every time you feel you're good enough to the point where you feel you can do better.