I believe Libya attempted to bring rain to parts of the Sahara by anchoring sand dunes and planting them. To anchor the dunes, they sprayed the slip face with crude oil. This would probably work if you could cover a large enough area.
(I've seen massive storms over the desert where the rain never reached the ground - I assume the heat radiating from the ground was sufficient to re-evaporate the water thus making the storm grow still larger - I estimated the tops as being at over 80,000 ft.)