Maple Story Doesn't Work/Doesn't Run Smoothly...

Check the system requirements.
The system requirements listed are minimum requirements. Minimum requirements are only enough to get the program running. For better performance, get a system that at least doubles the requirements.
Close all other programs.
Run virus scans and check for spyware, adware, and other programs that secretly run in the background.
Occasionally, Maple Story is updated. This means download the latest version or patch, and wait for the update to finish.
~One more thing... though it has already been mentioned. After a while your RAM memory fills with loaded screens, character info etc... if you find yourself disconnecting or lagging heavily try shutting down the computer to empty the RAM, then reboot it. That usually helps
http://strategywiki.org/wiki/MapleStory/FAQs