Best practice, setup 2 (or more) NTP hosts at your location, peer them. Have them sync against at least 4 (preferably, up to 8) external servers from 0.pool.ntp.org to 3.pool.ntp.org. If you use more than 4 you should adjust the frequency that they poll the pool members.
Here's an edited version of my ntp.conf:
server 0.us.pool.ntp.org minpoll 8 maxpoll 14
server 1.us.pool.ntp.org minpoll 8 maxpoll 14
server 2.us.pool.ntp.org minpoll 8 maxpoll 14
server 3.us.pool.ntp.org minpoll 8 maxpoll 14
logconfig +sysall +syncall
You can omit the minpoll and maxpoll arguments, I add them so I'm a bit lighter on those servers. The values are 2^n seconds, where n is the argument; those values are higher than the defaults (6 & 10) because I already poll 12 different servers between my three NTP hosts.
If you're very concerned with accuracy you might add the following as well:
server tick.usno.navy.mil prefer minpoll 10 maxpoll 16
This will poll the navy's atomic clock. Note the high poll times as they're fairly heavily loaded and have requested people take it easy on their server (actually a 3 node cluster).