Lifting weights is perfect for this. If you switch your grip between regular grip and "monkey" grip, you will impact your strength measurably. Use a reg. grip on everything you lift/push overhead, and monkey grip whatever you pull toward you. Because a great deal of your hand strength is affected by tendon strength, you will have to be consistent with this development and patient. The muscles in your hands will develop within months, but tendons take about a year or more to really, really get strong.