How to balance an attitude of "effortless non-striving" with proper posture?

In Karma-Chagme's book, "A Spacious Path to Freedom", he emphasizes the importance of correct posture in directing the vital energies to the central channel. In particular, he mentions (1) the tongue pressed against the pallet, (2) the teeth lightly touching, & (3) gaze directed 4 finger lengths beyond the tip of the nose. I find that when I try to incorporate these three elements of posture into my practice I quickly store up tension in my jaw and throughout my face in general. Additionally, keeping the gaze correctly placed is a full-time task that tends to excite the vital energies rather than relax the body. How can I balance keeping a correct posture while maintaining a relaxed attitude of effortless non-striving?