Frozen Tropics has an excellent list of Halloween tips. She does cover the disappointing habit of some kids to not wear costumes in points 4 & 5.
She also states that because Halloween falls on a Monday there might be some early trick or treating on the weekend. I'm ignoring those days, and I'll probably keep my normally on for safety porch light, off.
Last year the kids started coming around starting at 6:30pm and I ran out at about 8:30pm. If your block is well lit and there are other houses giving away candy, kids will come. Cute kids in costume will show up first, then as the night drags on, the uncostumed teens come and that's a sign that you should consider closing shop.
This year I'm going to be a chef, again. I have the toque and an apron. Other years I've been a nun, which is just a headdress of some white and black fabric and just dressing plainly. Occupations are the easiest costumes, I think. Get scrubs and you can be anything in the medical profession. I saw lab coats for sale in the UMD-CP book store, so I'll assume other colleges/university bookstores in DC have them, so there is the start for a mad scientist (just mess up your hair). Got glasses, and are your female with long hair? Be an outdated, stereotypical librarian by pushing the glasses to your nose, putting your hair in bun, and shussh people. Extra bonus if you have a sweater covered in cat hair. Got a business suit? Attach a bunch of legal sized paper to it and you're a lawsuit. Get it? Without the paper you can be a lawyer, or lobbyist. Slap a campaign button on your label and you're a politican. Carry around a folder with a bunch of resumes and you're a job seeker. Don't have a suit? Nothing but casual clothing? Get a stick and posterboard and you can be an Occupy DC participant and pick a cause, any cause, maybe demanding jobs and candy would be good one.
But regardless of these ideas, there will still be those kids who won't bother trying. They'll just show up at your door demanding handouts. Thinking that they'll grow up with that same mind set of not making an effort and expecting the same rewards as those who do, is scary.