The best way to keep chicken juicy when stir-frying is not to overcook it. You want the heat high enough that the chicken is just cooked through when it starts to brown. That way you get juicy chicken, but still with the browned edges, which add flavor. Then, you take it out of the pan immediately while you cook the rest of the ingredients. If you continue to add the other proteins and vegetables with the cooked chicken in the pan, you’ll overcook the chicken so that it is dry and not juicy.