1. <compilation debug="false">. This reduces overhead and increases performance and caching.
 
2. Turn on custom errors via <customErrors mode="on" or remoteOnly.
 
3. Disable or at least limit tracing info. <trace mostRecent="true" enabled="true" requestLimit="1000" pageOutput="false" localOnly="true"/>
 
Alternatively, this can be put in the machine config to enforce all the above for all sites on the server:
<configuration>
<system.web>
<deployment retail="true"
</system.web>
</configuration>
 
4. Enable Error logging. Elmah works great and is easy to integrate.