Heavy Bag work should be the main way that you build up your shoulders as a boxer.

Here are my tips

• Avoid heavy seated presses
• Use bands and medicine balls
• Train movements not muscles

Isometric punch -30 sec hold, and 1 sec pulses

Resistance Band Thrusters

Core Band Press

I personally like to do shoulder bridges and shoulder bridge presses, as I feel it stretches my core and hip flexors effectively, whilst building my triceps and deltoids.