You have to go to Sporcle.com and get addicted to the quizzes, like I did (work has been a bit slow lately). There's a ton of different ones you can take (naming the U.S. presidents, naming the U.S. states in alphabetical order, naming four-letter movies, etc.) but I challenge you on the countries of the world one.
The first time I took the quiz, I think I named about 60 countries. Then I decided that was shameful and have been working at it ever since, taking the quiz every few days and then studying the ones I missed. Now I can even spell "Djibouti" (a country in Africa), "Uzbekistan" and "Bosnia and Herzegovina" (that's a tricky one because it won't accept just "Bosnia")
Here's your challenge: go take the quiz, report your score in the comments and then assure me I'm now not the only one totally addicted to that site.