Without conducting an entire Art History lecture, I think the most concise and helpful advice would be for one to study the best among the full sized portraiture busts by the masters. In this instance, the realist artists past and present as it will relate to our hobby of literal depiction. Ditto 3/4 view portrait paintings and drawings.
You will see that the emphasis is on capturing not only a full likeness of a real human being in time and place, but also tweaking the subtle details that make all the difference such as the angle of the head, expressions, blending of the cropped shoulders, animation indicators of what is going on with the undepicted body below as it impacts the shoulders, upper arms, back, clavicle, and related adornment details all so that one is not distracted by the cut off point and leaves the viewer fully satiated. A wooden, looking dead on pose is avoided at all costs, a problem I refer to as the static, tobacco store Indian chief sculpts. The sculpt should appear alive before the painting has even begun on the very best of the bunch.
This is probably the most difficult form of what is already the acme of art, the human figure and face. If you are going to tackle portraiture, you had better have your chops in order. The finest portraits, busts or otherwise, have two key components: they capture a likeness of course, but they also have a je ne sais quoi that makes them stand out as interesting and beautiful works of art apart from the likeness alone.
As I mentioned, what may work for a whole figure, will be lacking in many cases with all the attention directed to the human face alone.
Additionally, someone mentioned that many scale busts leave one unsatisfied for what it is, rather they are left wishing that the whole, full length figure had been depicted. I concur completely. A well done bust won't solicit that reaction.
Again, this isn't a slam against busts in general by any means, rather it is directed against the caliber and quality as works of art of many of the offerings flooding the current demand.
At the end of the day, this hobby is about the uniforms and the individuals who wear/wore them. We as a group are definitely fascinated by the more material culture side of military history. I think in many instances, a full length figure is better suited to convey this.