Source: http://www.firefly.org/how-you-can-help.html
Create water features in your landscape.
Most species of fireflies have one thing in common: they thrive around standing water and marshy areas. Ponds, streams and rivers can all provide good habitats for fireflies, but even a small depression full of water can cause them to congregate. Build a small pond or divert a small stream to run through your property, and it’s more likely you’ll see fireflies at night. Chemically treated swimming pools aren’t a good substitute; fireflies are believed to eat the smaller insects, grubs and snails that thrive in natural ponds and streams, and these don’t live in chlorinated environments.
Cool cool cool!!!
But you also must not use algaecide or other toxins that will kill the same type of life-forms.