I'm planning to do my Dev 401 exam. I have been working in Salesforce platform for an year and would like to know what study materials I should go through and study before doing the exam. I would also like to know any useful tips and preparation questions, mock question.. where can I find them ? thanks !
That is a very broad combination of questions.
The official study guide made available by salesforce should be your base and give you an idea of the scope of the material you should know before taking the exam. It can easily be found on the certification website.
I gave a presentation at a local Developer User Group last month on this topic, which you can find on my blog here:
There is the slide deck and a talk through of what I covered. Stack Exchange is really for more technical questions though, so I fear this may be closed off soon anyway. Good luck with the exam!
The main thing is Force.com Fundamentals. That is totally enough. It covers all basic aspects of Force.com platform.
After that (or during breaks) you can explore dumps. Try to find dumps where an answer is hidden, so you try to find an answer by yourself (using Force.com Fundamentals for example).