Masturbation does not cause pimples.What you are doing is completely healthy and has no relationship with pimples.Pimples develop due to certain changes in your skin contributed by hormones to a certain extent.Hormones increase secretions in pilosebaceous gland which are broken down by bacteria to form an oily skin to cause pimples.Use a soap with minimal chemical, like dove with extra moisture.You need to drink plenty of water,exercise to reduce your fats and hormones,eat a balanced diet with lot of fibre,but less sugar and fat.Cream of Isoretino A(only after testing on skin to make sure it is hypo allergic,and cream clindac(clindamycin) twice daily after cleaning face will give you good results. Treatment is required for a month at least to see noticeable changes.