how do i download every purchase i've ever made?

There are many purchases that I've made on my iphone and ipod since 2007, some as recent as 2011. I've looked at my iphone settings to determine this. How can I download all of my past purchases? Why does Apple make it so hard to do so? If they are all related to my name and account, can anyone provide steps for me to download them ALL, not just 2012/13 purchases?