My Husband is a personal trainer and recommends using compound exercises (those that use multi joints) for building muscle mass. Exercises such as shoulder press are great for this. Varying up the workout by using dumbbells for a couple of weeks then changing to barbell changes the way the shoulder muscles are engaged. Aim for about three to four sets of six to eight repetitions with a sixty second rest between setes. If you are under 18 you shouldn't go below 10 repetitions because your bone's growth plates are still growing and this can cause significant problems later down the track.
If you really want to be mean to the shoulders try drop sets (i.e. do your normal set, drop the weight a little and complete another set straight after). Another common way of training the muscles a little harder is to do Super sets. Here you are just doing one set of two exercises back to back without rest (take your rest after you have done one set of each). Here you can choose to target the same muscle groups (lateral raise then shoulder press) or opposite muscle groups (frontal raise with rear deltoid flyes). This type of training works really well when doing split programs (different muscles on different days).
Don't forget genetics will play a large role in how difficult it is to achieve your goals and also diet is important. All the best of luck and don'y forget to train your other muscles equally as hard. Just because your not working your shoulders doesn't mean they won't benefit from doing other groups such as legs. Be sure to leave at least 36hours between resistance training the same muscle group for adequate recovery (some people need more). Hope this helps.
