Getting ImageMagick onto your computer would be so incredibly difficult I would often just forgo the entire thing and pray that image manipulation would sort of work on the server. Well, no more. Claudio Poli has written a great bash script to install the whole thing with one command. Clone this repo and execute it. Works perfectly!

Afterward I had to add this to my Rails development.rb file:
Paperclip.options[:command_path] = "/usr/local/bin"