Self studying for the ITIL Foundation exam is a method which is entirely possible- in fact, if the proper techniques are applied, one should have no problem passing the ITIL Foundation. Before diving in to ITIL practice exams, it is extremely important that you have a strong understanding of the content by which you'll be tested- this will differ between ITIL V2 and ITIL V3 exam takers.
For ITIL V2 exam takers, keep in mind that your ITIL Foundation exam will be based almost solely on the processes of the Service Support and Service Delivery ITIL volumes. One of the best texts to acquire which provides a great understanding of these ITIL processes is
"Foundations of IT Service Management: based on ITIL". Following a good read through and mind-mapping of this text, finding some good ITIL practice exam content is your next step.
For ITIL V3 exam takers, summarized content regarding the new ITIL core volumes are still limited, but you may want to check out It Service Management an Introduction Based on ISO 20000 and Itil V3. In addition, there are a number of ITIL V3 online courses which offer content for study. ITIL Prime offers ITIL V3 practice exams which can be used to solidify your studies prior to taking the live ITIL Foundation exam.