Conditional Where Statement

I have a stored procedure that performs a search function with params:

@username nvarchar(50)
@country nvarchar(50)
and like 10 more.

A user may provide values for these params optionally.
So when the @username var is left blank, there should be no filtering on the username field (every field should be selected regardless of the username)
Currently my statement is:

select username,country from myUsers where
username=@username and country=@country


With this statement when a user provides no value for username the username field selects on ''m which returns ofcourse nothing...

What can I do to solve this?

Thanks!