I've given tips on passing the Certification Exams in this post, based on the exams I've taken as well as for parts I contributed to past exams.
My main recommendations are:
- Get trained.
- Ideally spend a year with SDL Tridion.
- Be very careful with Tridion-specific terms.
- Versions matter (e.g. Taxonomy changes what you can do with Content Delivery APIs).
- Knowing Tridion functionality can help.*
*The interfaces (mostly) use the same APIs you do, so when in doubt, knowing BluePrinting, the Content Manager Explorer, and Publishing can help answer technical questions.
General test-taking tips:
- Read questions carefully.
- Take your time, but don't get stuck on one question.
- Improve your odds by eliminating wrong statements.
- Be careful with adjectives such as all, nothing, never, and only (the presence of these words don't mean certain answers are necessarily wrong).
- Don't over-think the exam.
Depending on the version of the exam consider studying the technical details related to new features. For example, SDL Tridion 2013 added Bundles and External Content Libraries, so you should expect questions related to these topics. You might find questions on Experience Manager but not SiteEdit.
Finally, good exam questions need plausible, but incorrect alternative answers. So it relies on assumptions, misconceptions, and things that have changed. In that sense, Tridion Stack Exchange is a good way to study as well.