I'm delighted to have earned the right to call myself a 'Certified Force.com Developer' today so I thought I would share what I consider to be some of the key aspects of preparing for the exam.
A lot of the core content is similar to the Certified Administrator, particularly around security and 'who sees what' - profiles, permission sets, org wide defaults, roles, sharing rules and so on. The 'Development' aspects are mainly concerned with the declarative capabilities of the platform, i.e. what can be configured with clicks. That focusses on Workflow, Validation Rules and Approval Processes. You don't need to be a programmer to pass this exam - knowledge of Apex and Visualforce only extends to 'Use Cases', there is no requirement to be able to write Apex or Visualforce code.
The full Study Guide is available here.
As for preparation, Trailhead and the Force.com Workbook got me through. ' Developer Trail - Beginner ' covers a lot of the material in the Study Guide. Work through the exercises in each whilst remaining cogniscent of the subjects in the Study Guide.
Working through those resources gave me a lot of new and useful knowledge, some of it in areas not required for the exam but useful nonetheless, such as configuring the Salesforce1 mobile interface and also Visual Workflow. The ' Cloud Flow Workbook ' is next on my hitlist.
If you already have the Admin Certification, Developer shouldn't be a stretch. Experience is key - get your own (free) Developer Environment , complete the challenges in Trailhead, visit the pages described in the text, press buttons and do stuff!
Prepare - Play - Pass - Go do it!