• Don't use pesticides
• Reduce or eliminate outdoor lighting
• Create habitat

1 is a no-brainer.

2: Artificial lighting will confuse lightning bugs or prevent them from mating, since females need to see that fancy swoopy-light dance to find a mate. When they don't mate, you lose out on the next generation.

3: I've had success in my yard because I've carved garden beds out in my lawn where I've planted a lot of different native plants, and I try to only minimally disturb the beds throughout the year, so lightning bug larvae have free reign in the leaf litter in fall and winter