Collocations for sag - verb

a bit (esp. BrE), slightly
visibly
She seemed to visibly sag at the thought of what lay ahead.

limply
All she could do was sag limply in his arms.

back, down, forward
Martin sighed and sagged back in his chair.
Helga's body sagged forward.

against
She sagged against the door.

under
The shelf sagged under the weight of hundreds of volumes.

sag in the middle
a mattress that was beginning to sag in the middle

Sag is used with these nouns as the subject:
mouth, shoulder, skin