Shoulder shaping on a simple shawl

I would like to add a bit of shoulder shaping to a rectangular shawl to keep it from slipping. Should I increase on both sides of a stitch at each shoulder point or decrease? I am willing to modify the shape as long as it is simple.:wink:

I think I would decrease to try to bring it in around the shoulders. I think increasing would make it want to slip off more.:think: