Go organic. Pesticides, herbicides and fungicides are poisonous and don't just kill the organisms they are meant to. Organic gardening results in a less toxic, more balanced garden that is better for all wildlife including bumblebees.
Plant suitable flowers and choose species native to your region if possible. Bumblebees are attracted to large clusters of nectar bearing flowers including lavender, Joe Pye weed, calamint, snapdragons and salvia. Plant a variety of flowers that will flower throughout the spring, summer and early fall.
Put up bumblebee boxes. Bumblebees need places to nest, and with less natural habitat, a shortage of suitable nest sites are available for them. Mount a few boxes in quiet places in your garden.
Create a woodpile. A large, loose pile of scrap wood, woody prunings and branches makes a home for all kinds of garden wildlife as well as bumblebees. Just don't burn it as a bonfire later.