The first and most important tip (that applies to attracting all other beneficial insects as well) is to make sure you don’t spray your garden with pesticides, which harm the good bugs along with the bad ones.
Make sure you grow a variety of nectar-rich, high pollen-producing flowers, particularly those that bloom in spring and fall so you get early season and late season aphid control.
Hoverflies prefer simple flowers (especially white and yellow varieties and clusters of tiny blooms) that provide easy access to food.
If you get these plants from a nursery, be sure the nursery can guarantee that the plants are free of neonicotinoids.
Starting from seed warrants some caution as well: Neonics are predominantly used as seed coatings, and these systemic insecticides can work their way into all parts of the plants (including their pollen and nectar) as they grow.