The best way to keep a bird from biting, nipping or otherwise acting out while on your shoulder is by avoiding letting your bird up there in the first place. 

Now, this can be challenging in and of itself! 

Here are some avoidance techniques: 

• Hold your bird's feet while on your hand so it can't walk up your arm
• Hold your arm position in one that puts your elbow-to-wrist below your shoulder-height. 
• Touch train your bird so that you don't have to "force" your bird off your shoulder.
• Set boundaries and stick to them! If you start off not allowing your bird up on your shoulder, but then do it "every so often", it doesn't paint a clear picture to your bird. 
• Offer something better; this could be a favorite toy, favorite person's attention, meal time, a treat (aka luring) 

Aside from continuing to avoid letting Morgan onto my shoulder, and keeping that a hard limit, I've also been expanding on her flight training. 

I was really stuck where I was at with how far of a flight I could ask for; I was already maxed out. So being able to get her to fly in a new direction made all the difference for me in regards to being able to expand on her flight distances (which will build more muscle, skills, stamina, and more confidence!)