The Golden rule of Compression (above all others) is this:
Try to compress your video only once. Each time you compress your product, you're taking a chance and lowering it's final quality.
I keep seeing people use tools like Handbrake to rip a DVD into MPG4 and then use another tool, like Apple Compressor to make ProRes. That's taking a distribution quality codec (MPEG2) taking it to another distribution quality (h.264) and then to an editorial codec.
Try, when possible to only transform your video once - at the end of your editorial process.
Compression Tips like this can be found by attending sessions at Post Production World at NAB (this link includes links to non-PPW sessions)