As a general rule, you won't stay out partying all night unless you're in Hongdae or Itaewon. Enough of those places stay open until the subways start up again, but few places in other areas will. If you're deadset on taking the first morning subway home, however, it can be done. Another thing to remember: most places will have tables and booths for groups. A couple will be fine, but you'll have a better time with a larger group - say, 4 to 6 people.
When it comes to partying, Hongdae or Itaewon are the two areas that come to mind. They cater to foreigners, offer cheap drink specials, and are the places to see lots of other foreigners. If you're tired of being catered to, however, here are 5 areas where the locals go to get their party on.