If you have a Series 1 TiVo, you can hack the remote so that you can push a single button and have it skip forward 30 seconds... conveniently, this is also the amount of time of a standard commercial.

Turn on your TiVo and TV and start watching TV. Then press the following sequence of keys on your TiVo remote: Select, Play, Select, 3, 0, Select. If you enter the sequence correctly, your TiVo will chime three times.

Once that's done, you can use the Advance button on your remote (the one with the play arrow that has the vertical bar on the right side) and your TiVo will skip forward 30 seconds, allowing you to easily skip commercials.