This is a simple way to make yasnippet and autocomplete work together on emacs. Both of them use the Tab key to activate. Everytime you press Tab, yasnippet will run first. If there is no snippet found, autocomplete will be activated.

Download yasnippet and autocomplete and add them to the load path or just simply install them using package.el. After that, add this to your .emacs

First, activate yasnippet.

;;; yasnippet
;;; should be loaded before auto complete so that they can work together
(require 'yasnippet)
(yas-global-mode 1)

Next, activate and config autocomplete. Put it after the code activating yasnippet.

;;; auto complete mod
;;; should be loaded after yasnippet so that they can work together
(require 'auto-complete-config)
(add-to-list 'ac-dictionary-directories "~/.emacs.d/ac-dict")
;;; set the trigger key so that it can work together with yasnippet on tab key,
;;; if the word exists in yasnippet, pressing tab will cause yasnippet to
;;; activate, otherwise, auto-complete will
(ac-set-trigger-key "TAB")
(ac-set-trigger-key "<tab>")

Restart emacs or eval-region to apply changes.