Plant plenty of nectar-bearing flowers and bushes they like. Butterfly Bush (Buddleiea davidii) being an obvious choice.
And make sure you have plants that the butterflies use as food for their caterpillars. Depending where you are there are various species that are worth growing. In sub-tropical and tropical countries where the Monarch butterfly lives plant any species of Milkweed or the Swan Plant. Butterflies like to sip nectar from flowers in this family too. For White butterflies plant Nasturtiums. For butterflies in the Browns family such as the Meadow Brown and Speckled Wood leave patches of long grass, and for Small Tortoiseshells, Peacocks and Red Admirals have a patch of Stinging Nettles. The Comma and Painted Lady will also use Nettles as a foodplant.
Painted Ladies also like Thistles, species of Mallow and the Hollyhock.
Many species of Blue butterfly need clover and trefoil species to feed their caterpillars and can survive on lawns as long as they contain these plants and are not mowed too short.
have a look and see what species of butterfly live in your area and then have a look on a website or in a book to find out what plants they need and plant these in your garden.