I prefer the self-check out and really hate some stores that limit the number of items you can scan on those lanes.
The old wives' tale is to look for a male cashier. Supposedly a male cashier is actually a management trainee so they are going to be more professional and knowledgeable.
I personally look for a cashier who is busy and not talking to other cashiers or employees. Some cashiers get so into their conversation with their co-workers, they don't stop when they have a customer. It is almost like the customer is an interruption to their day.
Other than that, my recommendation is to time when you shop. It might be a regional thing but I quickly learned not to go to the store in the hour after school lets out. OMG, there are so many moms and kids, it is nearly impossible to find anything much less check out.