I am a big fan of anything that gives me more insight as to what is happening on my system. One such IIS 6 tweak which I find is greatly overlooked is the additional AppPool logging you can get out of IIS 6. It blows me away that this was not "on" by default -- but that is another blog entry.
The tweek involves modifying the metabase per http://support.microsoft.com/kb/332088 to turn on this additional logging at the AppPool. This KB has a command:
cscript adsutil.vbs Set w3svc/AppPools/DefaultAppPool/LogEventOnRecycle 255
This turns the logging on for the DefaultAppPool however a more global approach, and one that would be useful esp. when you have more than one AppPool like us SharePoint guys is:
cscript adsutil.vbs Set w3svc/AppPools/LogEventOnRecycle 255
It goes without saying but after you set this you want to do some kind of monitoring for these events to get a since as to what is going on with your AppPools throughout the day.