This was the hallmark topic at a Linked-in piano forum board, with many piano teachers chiming in with great suggestions.
For me, singing beside a student or nudging him/her to join in is always helpful. I focus on shaping phrases in this way while also enlisting conducting motions. Finally, insights into how harmonic rhythm or the flow of harmonies influence a line are explored. (as well as matters of form and structure, such as an analysis of the motif, its elaboration, and where parts of a phrase returns, and might be extended–or pieced out–unexpected twists of a phrase, etc. or harmonic suprises) The student should have a sense of where a phrase is going or culminating.(Dynamics)
Dynamics and shaping phrases belong together along with an examination of articulation or note groupings.
Naturally the era of the composition and performance practice are important underpinnings of phrasing.
Finally, the physical expression of phrases translated into supple, relaxed efforts traveling down the arms into flexible wrists has to be a focal part of the creative process, and can’t be overlooked.
In this lesson-in-progress I flesh out the aforementioned approaches with an adult student who’s practicing the Mozart Rondo movement, K. 545 from Sonata no. 16 in C Major: