How can i make my wcf secure...

hey all,




 i have a wcf that i want to make little more secure...
i have database with user name and passwords..
and i want only those users can use the wcf..


how can i do that?


i dont want to pass user name and password in every function thats really silly  
public void helloworld(string somethingNice ,string username, string password)


so how can i secure my wcf (in a really simple way...)
i dont need anything fancy.. just simple to do :-)


thanks alot !!


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