Leaf mold is like hair conditioner for your soil! - basically its best attribute is to help your soil retain water. According to some university studies, the addition of leaf mold increased water retention in soils by over 50%. Leaf mold also improves your soil's structure and creates a lovely habitat for soil life, including earthworms and healthy bacteria. Clever eh?

Interesting fact: Leaves are basically carbon

Top tips

Pile up your leaves in an unobtrusive corner of the garden (or you can put them in bags - but make sure you put aeration holes in it)
Speed up the leaves' rate of decomposition by running a lawn mower over the pile a few times.
Make sure they are kept damp
Turn your pile every few weeks with a garden fork or spade to hurry up decomposition