Have poor dirty plate etiquette
If someone's still eating, you shouldn't start taking away plates unless the customer has pushed the plate away. If you start clearing plates halfway through the meal, people feel rushed.
Complain about customers showing up just before close
Sure, it sucks when someone comes in 15 minutes to close and orders a bottle of wine, but sweeping the floor loudly and sending death stares isn't going to solve the problem. If the customer complains, it could easily get one of your coworkers fired. And there's plenty of side work to be done while those last customers enjoy their mussels.
Lack empathy toward all types of diners
A server who is only good at serving one type of person is not a good server at all.
99% of the problems happen in the last hour of service. The kitchen is running on steam, customers/staff are more likely to be a couple drinks in, and everyone just wants to get out of there. Which is why it's important to be most on-point during these final hours, and maybe just smoke a cigarette to chill out while that couple finishes their bottle of wine.
Up-sell or over-sell
You might bank a couple extra dollars in tips, but a customer who feels falsely sold to isn't going to come back. And you're probably going to get a better tip if the customer ordered the thing they'd most enjoy.
Also, mentioning the $9.99 specials first inclines the customer towards price shopping.
Dan Gentile is a staff writer on Thrillist's national food and drink team. His least favorite phrase in the world is "are you still working on that?". Follow him to not-quite-finished entrees at @Dannosphere.