Romana.m, Hueber is quite helpful. I used that book too. I also:
- completed a TestDAF Vorbereitungskurs - this was a 4 week preparation course which was quite useful, particularly for the shriftlich and mundlich components as they are more difficult to mark yourself;
- used 3 other preparation books - I borrowed them from people who had already done TestDAF. I worked slowly through all 4 books, which really helped a lot;
- used a base structure for the schriftlich component. I kept practising one schriftlich paper every 2 days for the month leading up to the exam, so that the structure was stuck in my mind. The structure included some impressive sounding phrases which set up each paragraph (including intro and conclusion) and could just be adapted for each topic;
- drafted a table filled with vocabulary and ideas relating to topics which seemed to pop up regularly in mundlich and schriftlich components;
- recorded my voice practising the mundlich component (always timed) and asked my German friends to correct it;
- practised the mundlich component with fellow language-school friends (again always timed);
- only read and listed to German for the months leading up to the exam - it helps to find a publication which interests you and read it every week. For me, it was Spiegel. The only time I spoke English during this lead up period was when speaking with friends and family from Australia.
For your reference, I was going to attach the vocab table which I drafted in preparation for the schriftlich and mundlich components, but unfortunately I am not allowed to upload a Word file.
The TestDAF exam is all about preparation and strategy. The topics which pop up are quite predictable - but even if a new unexpected topic arises, if your preparation is thorough, you will be able to handle it. Also, make sure you are ready for the timing constraints, as they can be tough if you are unprepared. Final piece of advice is to check how your testing venue will deliver the mundliche Pruefung. It is often a room of 40 people or so wearing headsets and all talking at the same time. You should know whether that is the case in order to prepare - it could be off-putting to hear others if you are not ready.
If you have any further questions, ask! Good luck.