// Somehow detect the current logging level here?
System.LoggingLevel level = LoggingLevel.FINEST;

if(level == LoggingLevel.FINEST) {
  // Extra logging code that may make additional SOQL calls, etc...
  // that you don't want during standard execution.
  System.debug(logginglevel.INFO, level);
}

// Normally, suppress messages
LoggingLevel myLogLevel = LoggingLevel.NONE;
System.debug(myLogLevel,'A debug message that does not interest me today');
...
LoggingLevel myLogLevel = LoggingLevel.DEBUG;
System.debug(myLogLevel,'A debug message that I want to see today');