At MacRumors, I've published a lot of tips on mass converting videos to make one's life much easier when mass-converting several videos at a time. In the article below, I present you them all in an edited, more verbose format.
1, Using batch files (or looping commands) with command-line applications
Some of the video converters (for example, Project-X or FFmpeg) (also) have a command-line interface (CLI). Should you want to mass-convert / mass-process a bunch of files in, say, a directory or of the same type, batch files can be of tremendous help.