In the words of a pop song I've heard on the radio, you gotta keep your head up,...and for the best resonance, intonation and projection, these words could not ring more true. Make sure that your neck is not bent too low (don't connect chin to chest) and that you are balancing your head freely on the top of your spine.. I like to imagine keeping a bit of space between my neck and my skull, lifting the crown of my head. Keep the collarbone open and lifted as well and shoulders open as much as possible. Thinking of the shoulder blades pulling down, as the neck lifts up, can give your posture a gentle vertical lift. These small 'tweaks' to posture can really aid in your projection and you will also look much more confident as well.
Expression in music is a combination of a number of factors: tonal color/shading, dynamics, articulation and nuance, vibrato: the list goes on and on. Rubato is another device that creates an expressive line. Literally, rubato is 'robbed time', meaning if you linger in a spot, then you need to move forward again to 'compensate'. Sometimes students who start playing more expressively only slow down: this is only a rallentando, not rubato. Make sure to tape record your practice so you can really listen to your ideas and make sure that you are creating an elastic phrase, with supple contours, rather than just a slower moment.