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.

.Net go live checklist

C#