1. Don't slump in your chair.
Sit on the front edge of the seat with one foot more forward to balance you securely as you sit tall. When you "go" for something, press your rump into the seat, and your forward foot into the ground. This should allow your spine to stretch freely and flexibly.
2. Make sure your upper back stays stretched and flexible.
Don't let the curve of your spine slump, and don't freeze in place, either.
3.. Be sure your head is balanced over your tailbone, chin down and floating.
It's "smooch de morte" (kiss of death) if you let your face drift forward while singing. If you do move forward, do it from your hips, not your shoulders.
4. Use your eyes.
"Talk" with your face just as expressively as you would standing.
5. Use your hands.
"Talk" with your hands. If you are holding a mic, make sure you do that correctly. (Another post soon about that.) Those of you who read this blog or are my "Power, Path & Performance" vocal students know that I caution against letting arms become "rib anchors".
Try these things and let me know how you do... your comments always welcome!