"Don't slouch" as Sheila and Nanren suggest is a very common one. "Don't hunch" as Nanren said also. It sounds a little harsher then "Don't slouch", but I remember being told this as a kid.

Chin up! Although this also often applies to emotions (be happy, don't be so depressed and miserable); it also sometimes applies literally to a slouching posture, especially in the realms of dance and combat training. There are more clear and literal ways of saying it (eg: back straight; don't slouch) but they're neither trendy nor metaphoric thus I don't think they qualify as idiomatic. I don't think there's any really popular English idioms that apply to this scenario.

Stand up (or sit up) straight and don't slouch.

Posture! Mind your posture. don't hunch, may be what you were looking for, but Sheila's slouch is better.