need help about Single Sign On in asp.net

Hello friends 
I am trying to use the concept of Single Sign On (SSO) in my asp.net application. 
for the purpose i have created a main domain say mydomain.com. 
now i have 2 subdomains sub1.mydomain.com & sub2.mydomain.com. 
I am running these very fine. But now I want to have a SSO for my domains. For the same have 
googled lot and found some stuff such as adding machine key to web.config and specifying domain names in cookies. 
But it is not at all working for me. when I specify the domain name for cookies i am unable to sign out. 
I am using FormsAuthentication in asp.net. 
Following is the code I have in my web.config.




<authentication mode="Forms"> <forms name=".ASPXFORMSAUTH&q ...