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.