Learn and practice proper posture and technique every day. Rest the harp against your right shoulder and wrap your right arm around the harp. In this position, the right hand is restricted to the high strings while your left hand is free to pluck the low strings. Sit with your back straight to give ample support for the harp.
Use only four fingers when playing the harp. You use your thumb, index, middle, and ring finger. The little finger is not used to pluck the strings. Do not try to stretch your hands too far on the harp. The farthest accepted interval between thumb and ring finger by most professional harpists is 10. This will help prevent your knuckle from flattening.
Imagine a small bird is in your hand when you play. Your hand should always be curled as if it is holding a small bird or cylinder. You may want to practice holding a small object in your hand while you pluck the strings to get a feel for the proper hand technique. You should never have to flatten your knuckles to pluck the string.
Vary the difficulty of pieces you practice every session. Practice both an easy song and a difficult song and focus on your hand technique.
Listen to the difference between a flat knuckle plucking the string and a curled knuckle plucking the string. Train your ear to recognize bad technique to further train the hand position to muscle memory.