Note: Plato - is a period of time during your training, when results do not increase, may even decrease a bit, while you are training with the same intensity.
Thus second season I started with thought - I have to break the Plato. Both of them - physical and psychological. Because I simply couldn't believe that I can do more than 30 pull-ups. It was enough to remember feelings after 15 pull-ups to understand that no way!
I've collected recommendations from Internet and my friends, and here are possible solutions: