Look at small containers first
Plant saucers, buckets, watering cans, pet bowls, decorative pots and folded tarpaulins all hold enough water for mosquitoes. Empty or turn them over after watering, or after rain.
Check drains and gutters
Roof gutters and open drains that do not flow can hold water in pockets for weeks. Clearing leaves and checking that water actually drains away makes a measurable difference.
Manage water you want to keep
Ponds and water features can be kept, but they need attention: moving water, suitable fish, or emptying and refilling on a schedule prevents them becoming a breeding site.