What safety measures should I take when operating a backyard aluminum furnace
Set this up in an area free of leaves and debris.

I recommend wearing safety gear: heat resistant goggles and fire-resistance clothing and gloves that will protect you from heat, especially when pouring.

Get a couple practice runs in pouring before using it so you get a feel for the best angle to hold in and learn what NOT to do. Even better, you could assemble something that would do the pouring for you. (I'm thinking of something along the lines of this, but it doesn't have to be as elaborate.)

You may also want to scrape the paint off the cans before putting them in. Otherwise, you'll  get a lot of crud from the paint in your molten aluminum.