I have a cast iron drain pipe leaking at a rubber coupling. Can I just replace that band seal myself?

A leaky no-hub coupling on cast iron usually means the rubber's failed or the clamps weren't torqued correctly. You can replace it yourself with a new coupling and a nut driver, but you *must* ensure the heavy cast iron pipes are fully supported before loosening anything. If it's a main drain, or you're unsure about supporting the line, call a licensed plumber. Improper support can lead to a much bigger, more expensive problem than a drip.