This is what I have at the top of my ~/.emacs.d/init.el:

  (require 'package)
  (add-to-list 'package-archives
               '("marmalade" . "http://marmalade-repo.org/packages/") t)
  (package-initialize)
  (when (not package-archive-contents)
    (package-refresh-contents))
  (defvar my-packages  '(clojure-mode ...)) ; list of packages
  (dolist (p my-packages)
    (when (not (package-installed-p p))
      (package-install p)))