Photographing bats in flight is rather tough, especially without specialist equipment – like say a powerful floodlight or infra-red camera. My technique, which is being slowly refined with practice and experimentation, is to use the following tips with my Nikon D300 DSLR camera.

• The darkness and the speed with which the bats change direction will make it near impossible to use the viewfinder, so hold the camera up around your chest and aim as best you can. Review the shots regularly to hone your targeting, but beware of the LCD ruining your night vision.
• Use a lens with fair telephoto, but not so narrow a field of view that it's impossible to get the bats in frame, given you'll only be roughly aiming in the right direction. I've been using a 105mm lens, but I'd be tempted to use a slightly longer telephoto next time since I've got better at targeting.
• The bats will still be rather small in the frame even when only a few metres away, unless you're in the tropics and they're giant fruit bats, so expect to have to crop your results. A decent camera with a fair number of megapixels is helpful here.
• Use flash. You literally don't stand a chance without it and it doesn't seem to bother the bats. Be aware that it might bother other people though.
• Set to manual mode for aperture and shutter speed, with aperture wide open (f2.8 in my case) and shutter speed at least 1/200.
• Manually focus at about the distance you're seeing the bats and leave it set there. If it's more than 5m away then they're probably going to too small and dim in the frame though. The wide aperture will give very shallow depth of field so your subjects will be out of focus unless you catch them at just the right distance. This takes a lot of patience and experimentation, but autofocus doesn't stand a chance in the darkness and you can't see anything through the viewfinder so you have little choice. With a powerful flash and very high sensitivity settings you could probably use a smaller aperture for greater chance of non-blurry results.
• Use as high a sensitivity setting as is necessary in order to get a reasonably exposed result given the manual settings above. This can only really be determined by experimentation and you'll only be sure once you've got a bat in the frame. I varied between ISO 800 and 3200 so the results are going to be very noisy.
• Ideally position yourself so you can see the bats against the remaining light of the sky, but if that's not possible (they might be skimming a few inches above the water) or if it's so dark that you just can't see the bats with the naked eye at all then a powerful torch is vital.
• If the torch has a relatively narrow beam you can even tape it to your camera such that it shows where your lens is pointing, improving your overall accuracy.

Having done all of that, this is the best I managed shooting bats above a wooded lake in Hertfordshire. After endless shots of black featureless sky it's tremendously exciting to get something that's identifiably a bat! These are most likely pipistrelles of some variety.