Keep It Up -- Continue to plant container-grown trees, shrubs, perennial flowers, perennial herbs, and ground covers.
What To Plant -- Plant warm-season annual flowers and vegetables (tomatoes, peppers, marigolds, petunias and the like) and citrus and other subtropicals as long as all danger of frost has passed but in enough time for them to beat the summer heat. (However, in hot low-desert areas, for example, that can mean no later than then end of February.) If in doubt, give a quick call to the garden center nearest you.
Dividing Perennials -- Divide most perennials once they've sent up significant foliage at least a couple inches tall. Divide them if they are getting crowded (reduced blooms, a dead spot in the middle) or you simply want more plants.
Deadheading 101 -- Deadhead spent flower heads on spring-blooming bulbs to direct their energy back to their roots so they can build vigor for next year. However, if the bulbs were pre-chilled, don't bother. They won't, unfortunately, be returning in any case.
Fertilizers -- Fertilize roses and perennials and keep them watered as needed, especially new plantings.