(See I'm lagging! What should I do?.)
Measures can be taken to improve gameplay and help prevent an untimely death of a character:
Play on your fastest available network interface (such as Ethernet instead of wireless).
Close other processes (Internet, Skype) using your network.
Choose an uncrowded server close to you.
Ensure that you have the latest version of flash installed.
Flush your DNS. If you are using a Windows computer, hit Windows + R and type CMD in the run box that appears and hit enter. Type ipconfig /flushdns and hit enter. On a Mac, open Terminal and sudo killall -HUP mDNSResponder (10.8, 10.7) or sudo dscacheutil -flushcache (10.6). [This may help fix connection problems but probably not actual client–server lag.]
Clear your browser's cache [doubtful].
