First off, if you are not already an organic gardener then you should make the switch away from harmful, bee-killing herbicides and pesticides immediately. Using these harsh chemicals is harming the environment and local wildlife in a wide range of different ways. Moving to organic methods of gardening is the number one way that you can stop the decline in our bees.
Next off, you can make your garden more enticing to bees by planting a wide range of bee attracting flowers and shrubs that bloom throughout the growing season. Increasing biodiversity in your garden and thinking about planting bee friendly wild flowers etc. will make a big difference to the bees and other pollinating insects.
If you have a lawn, consider re-wilding that monoculture – clovers, dandelions and other 'weeds' are actually very beneficial for our bees. You may even like to consider getting rid of an area of lawn and creating a wildflower meadow in its place to create a bee paradise and improve your garden's biodiversity, making a more resilient ecosystem. Alternatively, consider creating a woodland garden – five blossoming trees could provide the same amount of pollen and nectar as an acre of meadow!
Another thing that gardeners can and should do to encourage bees to their gardens is to provide a 'bee hotel' – a place for solitary bees and other insects to live. You can buy many models ready made, though you can also simply make your own. Hollow sections of bamboo cane in a waterproof roofed box are a great alternative for those not able to keep their own hives.
Bees also need water to drink and to evaporate to cool their hives, but can drown when trying to drink from water bowls. A small bowl filled with pebbles to the surface will help them drink safely. Alternatively, a garden pond with pebbles or shingle on a very gently sloping bank at one side will enable bees to get their water without danger.