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There are many ways to increase friction between the ball and the rubber to make your service more spinny.

For example, for pimple-in rubbers, try contacting the surface of rubber (do not go so deep into the sponge) and the ball, and some people use more of their wrist to achieve this.  One of the other ways is to change for a more spinny rubber, such as Chinese sticky rubbers (DHS Skyline 2, Milky Way Apollo I, Haifu, 729, etc.)