The best way to tone your arms is to reduce fat and increase muscle size. To do this the National Academy of Sports Medicine recommends performing muscle endurance training and hypertrophy training. Muscular endurance is the ability to produce and maintain force production for prolonged periods of time. Muscular endurance training is exercises that have higher repetitions and moderate loads. Research has found that after the initial 12 weeks of training, multiple sets of up to 15 repetitions, 4 times a week for 6 months, resulted in a threefold decrease in body fat and an increase in local muscle endurance, as well as a significant increase in lean body mass.
Muscle hypertrophy is characterized by an increase in the cross-sectional area of individual muscle fibers, essentially making the muscles larger. To increase muscle size takes approximately 4-8 weeks to occur.