To begin, I should mention that this writeup is about how to catch a bat which has mistakenly wandered into your domicile. Unlike how to catch a snake, this is not about how to acquire a wild bat for fun and profit.

This writeup comes from my personal experience in successfully extracting a rat with wings from my apartment. By successful, I mean that neither the bat nor I was injured, and no damage came to the apartment. This was the method that was recommended to me by an animal control professional.

Preconditions

1. There is a bat inside

Step 1: Wait

If you have just realized that a bat is present, it is most likely night. Because bats are nocturnal animals, they are active at night, ergo more likely to be noticed. If you discover the winged mammal in the evening, I strongly suggest waiting until daytime to proceed. This will make the process easier for both you and the bat. If that's simply not feasible, this procedure may still work, but be careful.

Step 2: Gather Supplies

Now that it is daytime and the bat is (hopefully) asleep, you should be able to move about the area without disturbing it. You'll need at least the first two things for the bat removal, but I recommend three:

1. A Container. I recommend a piece of tupperware if you can find one big enough. This has the advantage of being clear which allows you to keep an eye on the little guy during the entire procedure. Other alternatives are coffee cans, or anything else suitably sized with a single planar opening.
2. A Cover. Cardboard works well, but anything thin and rigid which covers the container should do the trick. The animal control guy suggested using an LP. Opacity is only an issue if you have chosen a nontransparent container.
3. Gloves. Thick leather gloves work nicely, but anything that affords some protection without significantly hindering dexterity should work.

Step 3: Trap

Once you have everything together, the procedure is pretty simple. The bat is probably asleep and hanging from the ceiling. Make sure you have your protective gear on, and the container in one hand and the cover in the other. Stand on whatever you need to to reach it, then quickly place the container over the bat. Actually, smoothness is more important than speed, but don't take all day. It's more likely to freak out if you wake it up. Just be sure that you don't drop the cover. Now that it's trapped between the container and the ceiling, slowly slide the cover between the ceiling and the container. This is where a clear container comes in handy because it allows you to make sure you aren't pinching the bat's feet or otherwise hurting it. Make sure the container is completely covered, and remove it from the ceiling. Being careful not to tip the container, put your free hand on top of the cover so that the bat doesn't fly up and dislodge it. Congratulations, you've trapped the bat.

Step 4: Release

Now that you've trapped the bat, what do you do with it? Take it outside of course. Try to find someplace shady and cool. The animal is probably already agitated, so try not to make it worse by sticking it in direct sunlight. Once you've found your spot, set the container on the ground, remove the cover and step back. The bat should fly off pretty quickly. If not, leave and come back later. If it's still there, it may be injured, and you should call a veterinarian.

That's all there is to it. Now go wash the guano out of your container.

For the visually oriented: http://www-personal.engin.umich.edu/~baumanj/pictures.html#adventure. If including this link is poor form, /msg me and I'll remove it.