1. Going after a "hot" cert, rather than something you love doing. Many people transitioning into IT (and others in IT, but wishing to move into another area) want to pursue a certification based on its perceived popularity at the moment.
This is a bad strategy. For one thing, an area that's hot right now might be glacially cold in six months. These trends are impossible to predict.
The second reason it's harmful is that you may end up in an area you don't enjoy. If you like systems administration, why go after a developer cert? If you like getting your hands dirty with hardware, go after the A+ certification instead of a Cisco cert.
The point is to determine what you enjoy doing in IT and find a certification that will help you in that field.
2. Not getting enough experience first. Hiring managers have gotten more sophisticated about technical certifications over the years. When they see a certification, they wonder if a job candidate has commensurate experience. That's because some credentialed job-seekers try and use the certification as a substitute for experience.
Remember: there is no substitute for experience. Start at a low level if you're just getting into IT, such as working at a help desk or working for free helping out local charities; anything to get your hands dirty. Then your certification will have more weight, and your chances of getting that next job go up substantially.
3. Using one study method only. Books can be helpful certification prep tools. So can simulation labs. So can classroom-based training, and online training.
Don't confine your preparation methods to just one of these types. The book learning you get can help a lot once you get in a lab. Instructor-led training combined with video tutorials reinforces the concepts you're learning.
Variety is not only the spice of life, but it can kick your certification prep to the next level.
4. Testing before you're ready. It's a good idea to not have a hard deadline for getting your certification. In some cases, like needing it so your company can get a government contract, a deadline may be necessary. But in other cases, take your time.
You want to be thoroughly prepared for your exams before testing. If you feel you need an extra week or month -- or more -- be patient and delay your exam. If you don't have the concepts and hands-on knowledge down cold, it will hurt you on the test and in a live setting when the bullets are flying. Patience is a certification virtue.
5. Assuming certification will fast-track your career. Some people expect an immediate raise or promotion the moment the ink is dry on their credential. The more acronyms after their name (i.e. MCSE, CCIE, CISSP, RHCE, and so on), the more likely they are to feel that they deserve that corner office.
Certifications are one indication of your worthiness as an employee, but there are many others. They include experience, communication skills, ability to be a team player, etc. If you're not getting ahead the way you think you should be, ask your manager for an honest evaluation of what areas you need to shore up. Certification is no guarantee of success.
6. Using a braindump to prepare. Certification braindumps have been covered elsewhere on this site. For those unfamiliar, a braindump site collects exact questions and answers from certification tests and provides them to test-takers -- for a fee.
Braindumps are unethical to use; it's no different than getting a test ahead of time in school and regurgitating answers. It's cheating, plain and simple.
The other drawback is that getting a certification that way means you don't understand the material, so you'll be unprepared for your job duties if you do manage to get hired.
7. Not thoroughly researching training companies. There are hundreds of places to get certification training. They include technology vendors like Microsoft, Sun, Cisco and Oracle, to independent vendors like (ISC)2 and CompTIA, specific test-prep companies like Transcender and MeasureUp, boot camp companies, and more. The list is extensive.
Not all these companies are created equally. Do your homework before doing your homework, and you're much more likely to find the kind of training that fits your learning style and goals. Search out certification forums like the one on this site, and ask students what they liked. Talk to an instructor or two ahead of time. Ask for a list of graduates and talk to them.
Picking a training company is an important decision; don't take it lightly, and your chances of having a great training experience, leading to successful certification, increase greatly.