The commands need to be run at the command prompt with administrator privileges.
Windows
ipconfig /flushdns
Mac OSX 10.4 (Tiger)
lookupd -flushcache
Mac OSX 10.5/10.6 (Leopard/Snow Leopard)
dscacheutil -flushcache
Linux (Most Distributions)
sudo /etc/init.d/nscd restart
or
sudo /etc/init.d/networking restart
or
sudo /etc/init.d/networking restart