I really like Harry Barker Oatmeal Shampoo and Conditioner... Have used Yorkie Splash and Shine and like it too. I bathe Tillie and Bella every two weeks or so... depending on the weather... more often during summer of course
You may want to search
Shampoo in the box at the very bottom of the page (be sure to click YorkieTalk.com) as there a number threads with recommendations.