The single best thing you can do to create or improve habitat on your property is to start adding in plants that are adapted to your local area, and that have evolved with the pollinators and other wildlife you are trying to support. Indigenous plants will provide the best food and shelter for local wildlife. It's not just plants that create good habitat, it's the RIGHT plants. Many insect species require certain plants to survive. Think of the monarch butterfly, whose caterpillars only eat milkweed. Most caterpillars are picky eaters, and many bees are too. And the extra insects supported by native plants provide food for birds during the breeding season. In fact, birds breed the most successfully in yards where most of the vegetation is native.
Tip #2: Vary bloom time
When deciding what to plant in your garden, you want to make sure there is something providing food for pollen and nectar feeders throughout the whole growing season. Notice if there are periods where you have very little blooming, and look for plants that will fill in those gaps. Usually this means adding in more early spring flowers, like some of the beautiful spring ephemerals, or late fall plants, like goldenrod and other asters. Early spring flowers are crucial food sources for species who emerge when winter is just turning over into spring, and providing food late into fall not only supports resident species who are still active, but also provides migrating monarch butterflies with the energy they need for their journey south.
Tip #3: Vary bloom color and flower shape
The longer you garden with native plants, the more familiar you will become with the insects and other animals that visit them. You will likely observe early on that not all plants attract the same visitors. Hundreds of thousands of years of coevolution between plants and animals have resulted in some generalized patterns called pollination syndromes that can help you determine which flower visitors may be attracted to which types of flowers. Bees love yellow, blue, and white flowers, for example, while reds and oranges are more attractive to butterflies and hummingbirds. Some flowers have complicated morphology that might require a particular behavior or body size to access, so having both open bowl shaped flowers (like sunflower) as well as flowers with more concealed nectar and pollen, such as tubular flowers, will also ensure you provide food for a diverse suite of animals.
Tip #4: Provide both Nectar and Pollen Plants
Plants don't feed animals out of an act of altruism- they want to attract the best pollinators while sacrificing the least amount of energy (in the form of nectar and pollen) as possible. Each species has its own strategy for doing this. Some produce moderate amounts produce huge quantities of nectar to bring in abundant sugar hungry insects, but don't produce much pollen. Others produce abundant pollen to attract bees and other pollen feeders who are often more effective pollinators. And many have struck a balance, offering a moderate amount of each. Getting to know which plants provide which types of foods is important to providing nutritious forage for a variety of animals.
Tip #5: Provide Host Plants for Specialists
Some herbivores are generalists, eating a wide range of plants. Honey bees are a great example of this: they NEED to rely on a diverse array of plants to feed their large colonies and get adequate nutrition. However, others have adapted to feed on the leaves or pollen of a particular plant or a narrow range of plants. You're probably familiar with at least one example of this: the monarch butterfly, whose caterpillar only eats leaves of milkweed. If the host plant of a specialist insect isn't available, she can not feed her larvae something else. Therefore, providing plants whose leaves or pollen are food for specialist insects will create habitat for them while benefiting all the visitors in your garden.
Tip #6: Plant in clusters
Bees have a behavior called floral constancy- this means they tend to visit only collect pollen from one plant species at a time. Why do they do this? Think about an assembly line: doing the same task repeatedly is more efficient than switching between tasks. So bees focus on one type of flower at a time to collect pollen and nectar more efficiently. So having clusters of the same plant species in your garden can help bees maximize the volume of pollen they collect, while spending less energy flying from flower to flower.
Tip #7: Get comfortable with messiness
It is possible to garden with native plants and keep your garden looking neat and intentional. While a meadow or cottage style garden will have more density and diversity, you can also grow native plants in a more manicured aesthetic. But making a few compromises can help provide nesting habitat for bees. The majority of native bees nest in the soil, so be careful of what you put on the soil. Landscape fabrics and very thick mulch can prevent nesting or trap young bees underground. That doesn't mean you can't use it, just make sure you have some open areas for insects to burrow and nest. Another large group of bees nests in the dried stems from the previous year. Instead of cutting your perennials down to the ground in winter, prune them to a height of 6-18". As your plants come up in spring the new growth will hide the stalks, but they will provide homes for mason and leafcutter bees.