ps ax | grep lookupd | grep -v grep | perl -ne 'split; print $_[0]' | xargs sudo kill -HUP

I'm often connecting to different networks depending on what job I'm doing, and sometimes I'll need to tweak my /etc/hosts file. Since my system will cache DNS lookups, I'll need to restart lookupd, and this command does it in one hit.

Refresh DNS in Leopard

sudo dscacheutil -flushcache

Refresh DNS in Yosemite

sudo discoveryutil udnsflushcaches