In his PodCamp presentation entitled Gear Talk, David Berlind mentions that he uses his eyes over his ears when mixing audio signals to a consistent level.  That is, he watches the level indicators to ensure that the audio signals are averaging in the same range and as close to 0db as possible.

There is a significant danger in trusting your eyes over your ears with an audio program - your audience uses their ears to listen to your show.  So, while using your eyes is helpful, using your ears is imperative.

Here’s why: frequencies.

One audio source could have a rich blend of all of the frequencies in the audio spectrum; the other could be heavy in the high registers and lack any presence in the low and mid ranges.  The first source will be rich and can sound loud without actually being so.  Because it is so shrill, the second source can be extremely painful to listen to if mixed to the same level as the first source.

Check with your eyes, judge with your ears.