U.V. light traps
There are a great number of commercial designs for U.V. (ultraviolet or black light) insect traps, with increasingly sophisticated options available, such as designs that also emit CO2 for sampling mosquitoes. Light traps are useful for sampling a variety of insects, including nocturnally active flies, beetles, and various wasps, but they are especially effective for moths.
In our urban study, we trialled a cheap light trap design that consisted of a flourescent U.V. tube suspended over a white tray of water with a drop of detergent added to break surface tension. The lamp was run from a 12V battery housed in a plastic box to keep any rain off. The majority of insects attracted to the lamp ended up in the water and were collected in the morning. Of all the trapping methods we trialled, the U.V. light was by far the best for collecting moths.
If you prefer your moths are kept dry, then the lamp can run over a suitable container with a translucent funnel or angled baffle system at the top. The insects fall down the funnel or baffles into the container and cannot find their way back out.
A potential drawback for U.V. light traps are that they are relatively expensive for each unit and at risk from vandalism or theft when used in unsecured public spaces. If you do not wish to leave your light unattended, the cheapest option is merely to fit a fluorescent U.V. light tube to a standard flourescent lamp and run it over, or in front of, a white sheet. Moths and insects attracted to the light settle on the sheet where they can usually be collected by hand.