When I go to the grocery store, I grab my shopping cart from the outside corrals instead of from inside. That way it's one less the employee has to take back in.

Bonus: I'll also always put the cart in the stack of the same kind when I put it back - a lot of places have various-sized carts and most people just shove them in there haphazardly.

I've found most people who have worked retail tend to do little helpful things like that without even thinking about it.