What are the best practices for network design and implementation in order to have a cost effective and efficient network?

design multi function network and make it efficient using QOS and routing protocols and Access-lists.  

layered network architecture; with dedicated configuration on every layer to conserve processing  power.

access/edge switches to connect users and end devices

like to combine the distribution/core layers to connect edge/access switches as a cost effective solution and invest in redundancy in this layer.