Tuck your lower back in. Don't rest your weight on your arms. Try not to let your back muscles fall into disuse by leaning on the chair back. Whatever chair setup helps you achieve these is probably fine.

For me what works well is backless chairs that allow me to angle my thighs downward slightly and tuck my feet underneath me and to the side. Right now I'm on a yoga-ball-chair (basically a yoga ball that does not roll). Previously I sat on a large round ottoman.