I usually take and cut off the sunflower heads when they are drooping over. That way the birds can get the fill they want and I still get some to use for the next year.
However, cut them off with enough of the stem left so that you can tie them up. That way they will not be laying on any surface which could allow some pests to get to them. Then you can hang them up in a garage or other area to dry for a little while. That makes it easier to get the seeds out of the head.
Usually I let them dry for about a month and they can easily be pulled out or you can bend the head and break them out. (this way tends to be more messy though)!
Hope this helps you out.