[theenemy]

One tactic I like to use against rushers is the scout spam(maybe someone here has mentioned it already, I haven't read all posts). Like Baalz mentioned, rushers tend to be aggressive so they PROBABLY invest most of their resources into steamrolling you. They mostly don't care about PD.
So what I like to do is to build a lot of scouts and them into the rushers territory. Remember: An undefended province can be taken over even by a scout.
Then I buy some PD in the newly conquered provinces and laugh an evil laugh.
Hopefully this will get the rushers attention and make him turn back his armies to reconquer his provinces...unless of course he thinks that he can steamroll you anyway.
This is not some kind of perfect plan, but it has worked for me a couple of times.