How to stop my bag strap from slipping?

Hi all, I've just found this sub so apologies if it's the wrong place...

I recently bought a bag which has an adjustable canvas strap.

(Think https://ae01.alicdn.com/kf/HTB1xUhBIXXXXXboXXXXq6xXFXXX4/3-2cm-wide-bag-belt-20-colors-woven-shoulder-bag-belts-canvas-straps-DIY-bag-strap.jpg)

I adjusted it to the length I wanted and all seemed fine, but as soon as I put anything in the bag it slides down my torso and drops too low, bouncing on my thighs whenever I take a step.

Looking for a non-permanent solution to keep the strap at comfortable length which doesn't involve safety pins?

Thanks!