Design your garden so that there is a continuous succession of plants flowering from spring through fall. Check for the species or culticars best suited to your area and gradually replace lawn grass with flower beds.
Plant native to your region using plants that provide nectar for adults plus food for insect larvae, such as milkweed for monarchs. If you do use non-native plants, choose ones that don't spread easily, since these could become invasive.
Select old-fashioned varieties of flowers whenever possible because breeding has caused some modern blooms to lose their fragrance and/or the nectar/pollen needed to attract and feed pollinators.
Avoid pesticides, even so-called "natural" ones such as Bacillus thuringiensis (BT). If you must use them, use the most selective and least toxic ones and apply them at night when most pollinators aren't active.
Install 'houses' for bats and native bees. For example, use wood blocks with holes or small open patches of mud. As little as 12" across is sufficient for some bees.
Supply water for all wildlife. A dripping faucet or a suspended milk carton with a pinhole in the bottom is sufficient for some insects. Other wildlife need a small container of water.
Provide water for butterflies without letting it become a mosquito breeding area. Refill containers daily or bury a shallow plant saucer to its rim in a sunny area. fill it with coarse pine bark or stones and fill to overflowing with water