Really I'm also thinking about the same point, because I already have a private cloud environment built on 3 physical servers. And my next step is how to integrate SDN with OpenStack.

you can use opendaylight lithium with openstack 

If you do not know much about SDN, i suggest use some tutorial with openflow, mininet and some controllers.

This tutorial helped me a lot:

http://archive.openflow.org/wk/index.php/OpenFlow_Tutorial

Now i am studying POX.

My intention is begin with a machine (or virtual machine) and a load balancer, then use SDN for some allocation or balancing policy (mininet or POX , for example), and then redirect for some virtual machine in my private or public cloud.

And try to study some elasticity mechanism using SDN.

we are using OpenDayLight, an SDN implementation on OpenStack.

I am also working on the integration of Openstack with Opendaylight lithium. But there is a kind of hitch that Opendaylight does not provide good and clear guide to connect them.