1) Accept the risk.
First things first. This is not Disneyworld. These are real places in the real world. They have real ceilings that might cave in and real rusty nails that can stab you and real people, occasionally, living in them. So if you’re going to explore urban ruins, you need to accept that something bad might happen. Probably not, but it might. So be smart and proceed with caution.
2) Buddy up.
Any time you’re going to do something risky, it’s better to do it with a buddy. I was lucky enough to check out the ruins of Gary with a dude who’d filmed a zombie movie on location there (which, PS, add “guy who makes zombie movies” to the list of friends everyone should have, along with lawyer, nurse, and cousin of the alderman). So I had a somewhat knowledgeable guide to make sure we didn’t get lost in the psych-ward corridors of the old post office. It is not strictly necessary to make sure your buddy is a big, strong guy, but it helps. Either that, or someone slower than you who will become werewolf bait.
3) Tell somebody.
This is true of all travel. Make sure someone knows where you’re going and when you expect to be back. If you can get clearance from some kind of authority in advance (we had a permit from the film office), that’s nice, but at the very least, make sure that somebody back home knows that they should come looking for you tomorrow, because you’re maybe being tortured in a dripping, dank basement somewhere.
4) Be prepared.
Wear sturdy, closed-toe shoes. (You were thinking about wearing flip-flops, weren’t you? Dummy. You’ll get fourteen kinds of tetanus and break your ankle.) Also, long pants are a good idea, because a lot of these places are overgrown with a variety of brush, including poison ivy. Flashlights become really important in the inner hallways of buildings with no electricity, assuming the roof isn’t gone. Pack food and water, because any place with this many abandoned buildings isn’t likely to have a ton of lunch options. And also: Purell. Always a good idea.