The 2 things I see that cause most of the problems are

1. The people who put their bag in sideways without even trying to put it in wheels or handle first even though it is obvious it would fit.'

2. The people who bring more than 1 carry on an 1 personal item. I see more people with 3, 4, and sometimes 5 items and cram them all into the overhead.

Enforce the amount of carryons and enforce the wheels/handle first (yes I know there are some planes like the 76x that even normal size carry on don't fit that way) and there would be a lot more room for everyone to share.