From http://docs.shopify.com/support/your-store/7-collections/how-to-navigate-within-a-collection

+product.liquid

   {% if collection %}            
      {% if collection.previous_product %}
         {{ '« Previous Product' | link_to: collection.previous_product }}
      {% endif %}
      {% if collection.next_product %}
        {% if collection.previous_product %} | {% endif %}
        {{ 'Next Product »' | link_to: collection.next_product }}
      {% endif %}
   {% endif %}