Well, I've tried many different approaches. The one which works best for me is to read and understand some text and then listen to the corresponding CD over and over until I can understand without referring to the text or stopping the CD. This has also improved my ability to hear the tones. The book and CD 'teach yourself Vietnamese' by Dana Healy is quite good and there is a lot of very useful vocabulary in 'Vietnamese for Beginners' by Jake Catletta and Huong Nguyen. The 1st book is in the Northern dialect and the 2nd is in the Southern form.