uspravan

1. upright; erect; vertical
   Sedi uspravno, ne krivi leđa! — Sit up straight, don't hunch your back!
   Držao je glavu uspravnu i gledao pravo. — He kept his head upright and looked straight ahead.
2. (of a person, character) upright; honest; principled
   Moj deda je bio uspravan čovek do kraja života. — My grandfather was an upright man until the end of his life.

The adverb uspravno is common with stajati, sedeti, hodati.