Here's a little knock down example:

CREATE PROCEDURE jas

@lname CHAR(25)

AS

IF @lname IS NOT NULL
  BEGIN
	SELECT * FROM Client WHERE Lastname LIKE RTRIM(@lname) + '%'
  END

IF @lname IS NULL
  BEGIN
        SELECT * FROM Client
  END

GO

There might be a better way to do it but this is the way that I use.