Snort: Best Practices

[comprepsrv]

Hello again.  Now that I have snort running, what is 
the best configuration?  I notice a major slowdown on 
our 100 Mbps LAN.  What is the best ruleset or config 
for IDS?  All it seems to log are 'destination 
unreachable' and 'portscan' from the external nic, 
nothing else.

thanks

dp