I'm surprised that ELMAH is only mentioned in a comment so far. Frank may be familiar with log4j, why log4net may be an obvious choice. Be he is also asking what's the best practices for error logging in C# web applications. ELMAH is the de facto standard error logging component for .NET web applications (Microsoft themselves uses ELMAH as an example in pretty much everything they write about error logging in web applications).
I've written an ELMAH Tutorial, which show you how to get started.
Also if you've already fallen in love with log4net (nothing wrong with that), there's a log4net appender for ELMAH.