Less pesticides & artificial fertilisers
These not only degrade the nutrients that bees get from pollen but are also toxic to bees. If a bee pollinates a plant that contains pesticides or insecticides the bee inadvertently brings traces of these pesticides back to its hive, contaminating a whole colony of bees.
If you must use pesticides then opt for the emulsifiable concentrates rather than the powder variety.
Less pesticides & fertilisers will provide a better diet for bees, increasing bees overall health!

Let your grass & wildflowers grow!
Or, at least leave small sections of fields alone when cutting the grass to allow it to grow freely. 
Doing this creates areas for bees to nest and pollinate in peace and is also beneficial for the growth of dandelions & other wildflowers which are great food sources for pollinators.
Weeds are also beneficial but if you must remove them then pull them instead of using weedkiller.

Create bee nests
This can be achieved by creating your own bee box by drilling holes in wooden blocks or creating cavities in stone walls or cement structures. 
This will provide pollinators with a safe area to hibernate, breed & nest. It also necessary to ensure that there are flowers nearby.
Taking these measures can also accommodate for other insects that are beneficial for pest control on the farm.