Keep doing them. With good form. The better you do them the more "easy" they become. If you can do up to 10 of bodyweight pull ups, then I'd start adding some weight progressively loading the weight as you become more comfortable doing pull ups.
I had some injuries a while back and so couldn't do all the exercises I wanted so spent more time doing them, so I guess it just comes down to doing more but without fatiguing. Accessory exercises like rows, and so on are also helpful to build up that strength.