Does anybody in long-term relationships still flirt?

Yes you can still flirt with someone you've been fating for while it keeps things fun and intresting.

Umm I don't think you can? Flirting is before your dating, to drop the hint, you know?
when your already dating, you have to get more "fun" if you know what I mean.
Take her out to an amusement park or a carnival or somewhere fun. Ice skating is a really good place to have fun as well.
Trust me; fun, romantic places keep sparks way up high :)

It could be reached at a different level. While you certainly may know how she would react, you can still play those little games of fun. Mind you, I am not talking about those stupid games most people play around one another to figure each other out, but usually end up hurting themselves and the relationship.

It could be as simple as a subtle touch to her neck, or the back of the ear which she enjoys the most, while shopping at the mall. Your intention is to tease her and turn her on, knowing that she can't do anything about it at the moment. When she figures out what you are trying to do-and this is when she is turned on- she will give you a look of 'Are you doing, what I think you are doing', at which point, you can give her the same look with a smirk. That same girl, who loves to shop around for hours and hours now can't wait to finish it and go home right away.

This is just an example. It may work at times, while it may not work at others. Or you can improvise it to use it different scenarios.