They sweat and eat lean foods. Its fat that covers your muscles and prevents that definition ppl like so much. Lots of cardio: running, dancing or getting in saunas etc. Another trick is using light weights and lots of repetitions. For bicep definition; if your normal curling weight and regiment is say 30lbs-35lbs and 4-6 sets of 8-12 reps; try using 5 or 10 lbs on each arm and doing 2-4 sets of 50 or 100 reps.