To reset (aka flush) the DNS cache in Yosemite (OSX 10.10):
sudo discoveryutil udnsflushcaches
For historical sake, in Mavericks (10.9):
dscacheutil -flushcache; sudo killall -HUP mDNSResponder
And Lion/Mountain Lion (10.7/10.8):
sudo killall -HUP mDNSResponder
