Don't SPAM the logs.

You don't need to log every method call, every variable, every "if" statement, ect. That's what the debugger is for.

And, I shouldn't need to say it: Don't log passwords or other sensitive information.

Also: Make sure your logger is configured correctly. Log4Net used to (and it still might) calculate an entire stack trace with each call to the logger. It can be the single highest consumption of CPU in an application; even if you aren't actually logging stack traces.