Most minor injuries occur while walking downhill; knee strains are common, especially among trekkers carrying their own packs. If you know your knees are weak, bind them up with crepe (ace) bandages as a preventive measure, or hire a porter. Good, supportive boots reduce the risk of ankle sprains or twists, but the best prevention is just to pay careful attention to where you put your feet. A walking stick or hiking pole(s) can help, as can using Nepali technique: smaller, pitter-patter steps on steeper slopes.
It’s hard to avoid getting blisters, but make sure your boots are well broken in. Some people swear by two pairs of socks. Airing your feet and changing socks regularly also helps: you can have one pair on while the other dries on the back of your pack. Apply Vaseline to rubbing spots and protective padding (moleskin is recommended) to hot spots as soon as they develop, making sure to clean and cover blisters so they can heal as quickly as possible.