2 things from my first rookie mistake I made when buying the right gear when I bought my first new road bike:

1- buy the smaller of the two sizes if between sizes. I bought nice shorts that are just a bit to big and they fall down in back when riding. these things will stretch out. you want you fit tight, not constricting but very snug.

2-buy bibs, i bought a pair cause said shorts had poor fit and i am now converted. Will never buy a pair of shorts again. Bibs just work better for me. they stay up and in the right place no matter how long I ride. The LBS tried to send me in this direction and i pushed for the shorts, should have listened. They also said how they should fit but i wasn't use to the tight fit and went for the large instead of medium. dumb move

Lastly, I bought Louis Garneau Equipe bib shorts and they were more expensive then I would have hoped to pay but are worth every penny. The padding is far superior to the cannondale shorts I bought first. and they fit me better.