How can I lessen my milk supply?

Why? It's physiological reflex, which is regulated by the babies needs, his sucking abilities and hormonal response. As the baby growing, the milk production increases, and than slowly decreases towards the 12 month, in average.
Reduce feedings. Plan on decreasing your breastfeeding sessions by one a day over several weeks. This will enable a slow steady decrease in production and a more comfortable weaning period.