Advice on Clean Work

So I learned a little lesson the other day that was hammered into my mind...ALWAYS clean your ruler. I was really tired and cranking out a comic page and after penciling it, I looked down and gasped in horror...there were ink marks ALLLLLL over my paper. But I didn't have enough time to go back and clean it up so come critique time, my professor asked me if I dropped it or played hockey with it. Haha. As a comics/animation artist, stills and comic pages sell usually pretty well...unless they're dirty and have ink marks everywhere. So this is a lesson to me and I thought I'd save others the trouble by dropping this piece of advice into the interwebs. Sure, its common sense, but common sense isn't always that obvious...so to all you artists out there, clean your rulers and supplies and learn from my mistake.:)