Anti-Bush U.S. media bias?

A favorite trick on both the left and the right is to accuse The Media of being biased to the right or the left, respectively. How does one provide evidence for such an accusation? The way to do this is to research what information is available, and compare it to what information is being reported. If we could establish that news corporations regularly under-represent news that would tend to, say, make conservatives look good, and over-represent news that casts liberals in a positive light, then there might be a case to be made about bias. If we could establish that news corporations regularly reported false stories on behalf of a political group, or reported obviously false claims without reporting that they're obviously false, that would be an even stronger case. So allegations of media bias require a somewhat involved set of evidence to back up. This is not a trivial matter, but it can be done. It's definitely, definitely not so simple as observing that a majority of stories reported are critical of the President, therefore the media has a liberal bias. An unbiased media would call things as they are; if most of the President's newsworthy actions are such that pointing them out would be politically damaging, then an unbiased media's reports will reflect this.