Encouraging community collaboration (plugins / themes)

I’d like to present on various ways to encourage other members of the community to contribute back to your plugins / themes, in the form of code patches, assistance with support, and marketing (sharing the word).

I will present on a lot of the things I have learned from developing / managing Easy Digital Downloads, a popular ecommerce plugin that has a large number of official contributors and even more one-off contributions from members of the WordPress community.