For custom Rake tasks, you shouldn't need to modify them after the original definition. However, if you want to add behavior to some vendor tasks (such as those defined with Rails), this blog post will cover how to do that.

Growing Rails Applications in Practice

Check out our e-book. Learn to structure large Ruby on Rails codebases with the tools you already know and love.

Introduce design conventions for controllers and user-facing models
Create a system for growth
Build applications to last

Modifying Rake Tasks - Dan Manges's Blog

Growing Rails Applications in Practice