Free-tailed Bat, copyright 2011 Leonardo Ancillotto. All Rights Reserved.
Disclaimer: This applies to North American New England bats; if anyone very knowledgeable about bats of the world would like to comment and add advice, please do.
If a bat gets into your house, here is what to do:
This almost always happens at night, so try to isolate the bat in one room by turning the lights off or low in that room, and turning the lights on or bright in other rooms. Bats are not attracted to light. To isolate the room, close the doors very slowly and very carefully. This can take some constant effort because the bat will usually be swooping and dive-bombing throughout the house. Be very careful not to slam doors or barriers — you do not want a squished little bat.
I tend to sort of crawl around on the floor while doing this, to avoid the bat’s flight path. No, they are not going to dive for your hair or face — but they do swoop and dive, and if your hair or face is in the way, oh well.
Once the bat is isolated in one room, open a window — make sure the screen is up and open, preferably as you are opening the window; you don’t want the bat crashing into the screen repeatedly.
If the bat doesn’t immediately fly right out the window, turn the lights off completely, and it should head right out.
Done.
Okay, so that’s what to do. Here is what not to do:
Scream and run around like a nutter, flapping your arms.
Grab a pillowcase or towel or other item to trap the bat in. Bad idea. Hurts the bat, may hurt you.
So there it is, that’s about it. Simple.
Bats are largely beneficial creatures and they want to be in your house even less than you want them to be there. Be good to the bats and ease them out. Remember, “Don’t panic.”
Also, please see Bat World Sanctuary‘s page on What to do if You’ve Found a Bat.
YawningDogBlog 
Is that your photo? I’m really envious! I wish we had bats hereabouts to eat our mosquitoes.
“they do swoop and dive, and if your hair or face is in the way, oh well” Really? I had thought that bats were masters at maneuvering their flight and avoiding obstacles!
Ahh that’s such a cool shot!
They are good at it, but the best-laid plans of bats can sometimes go awry.
I love bats. Good thing I do, too, otherwise there would be no one to protect Nitram from them when they come in the house. :D
The photo is credited to a contact of mine on Flickr.
Oh sorry I didn’t read carefully!
Yay for bats!
Bats that are trapped indoors are sometimes disoriented and don’t fly as well as they normally would. Plus, they need to swoop occasionally in order to maintain their gravity in flight. This swopping can sometimes be mistaken for an attack is the person is standing in the spot where the bat needs to swoop.
Thanks for stopping by, Amanda!