Shoulders need to flexible and strong to stay injury free. Don't think that just by breakdancing you will train them properly. Make them stronger and more flexibile than you need then breakdancing will be that much easier.
I do the basics for shoulder flexibility as shown in the video. Shoulder flexibility is very important for specific moves such as hollowbacks and airchair, it becomes less important for flares and footwork. However, with greater flexibility more moves are possible. If you aren't spending at least 15 minutes a week stretching your shoulders you are wrong!
Strong shoulders make strong bboys. If you want to be on your hands and make it look easy start get comfortable being upside down. Handstand walking, handstand pushups and handstand holds are the basics you need to master. If you want to go to the next level start training like a gymnast and get upside down on rings.This video gives you a peek at how I maintain my shoulder strength to stay injury free and do all the moves I have in my head.