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If you haven’t shot the tape yet, try to maximize the strength of the desired sounds by putting the mics as close as possible to the sound sources. Turning up the audio gain will not work and will make the hum worse. Also, be sure that any directional mics are oriented as to minimize the hum pickup. Finally, turn off any unused lights. You might as well check for other sounds such as fans etc as well.
In post, the filter suggested by birdog is a good idea. This is not a filter in the classical sense. What you do is record some room sound which consists of only the noise and not any desired sound. The software then subtracts the digital pattern of the noise from the overall desired sound plus noise. I have done this in Adobe Soundbooth and it works pretty well. It is not perfect, however, so minimize the hum before recording it.
You can try a notch filter, also included in most audio editing software. You set it for the hum frequency (60Hz in the US). It will probably be necessary to apply several notch filters at the harmonics of the hum (120 Hz, 180 Hz, 240 Hz etc) because the hum you are hearing is not sinusoidal.
Finally, possibly the best solution is to add background music or other sounds to mask the hum.