Hi all
I found the solution , its very simble , I am using Sample Flex viewer that is by default supporting proxy , I just added the this tag to config.xml in flex viewer project :
<httpproxy>http:\\myserver\proxy.ashx<httpproxy>
then I added userproxy="true" attribute for each map layer in config.xml , add this only for layers you want to use it throw proxy , now SFV is ready to use proxy .
I have attached a proxy version that supports dynamic token (short lived tokens)
also I have enabled security in ArcGIS server manager , I used SQL authentication
thanks