General synonyms:
* Stooping
* Hunching
* Sagging
* Lolling
* Drooping
* Sloucing
* Slumping
* Caving in
* Rounding (one's shoulders)
More informal synonyms:
* Hanging (one's head)
* Slumping over
* Flopping
* Relaxing (in a bad way)
More formal synonyms:
* Posturing (if slouching is done intentionally to seem casual)
* Curving (for a curved spine)
* Deflecting (if slouching is done to avoid eye contact or being noticed)
The best synonym to use will depend on the context of the sentence.