How should I prepare for the CEPT entrance exam?

As you did not specify which exam you are preparing for it is difficult to give examination-oriented advice. 

The table below will give you a brief insight into the CEPT Examination. 

With CEPT you can take admission in Bachelors of Urban Design, Bachelors of Interior Design, and Bachelors of Construction Technology. You can also take admission in postgraduate courses such as Master of Landscape Architecture, Master of Technology in Structural Engineering Design, Master of Urban Management, and Master of Technology in Geometrics. 

Discussed below are general preparation tips that can be helpful.

Undergraduate Studies - BArch/Design/Engineering:

• Qualify NAT with good scores in Aptitude, Science, Mathematics, and Analytical Skills. 
• Prepare well for topics like Aptitude, Logical Reasoning, basic knowledge, and orientation towards the Architecture for CEPT. 
• The interview round will test your personality and skills.

Postgraduate - Masters in Architecture/Planning/Design/Technology:

• A good GATE score will be helpful.
• Good marks in Architecture shall help as the exam has objective and subjective type questions. But the selection depends on your skills. Do not read architecture books like pattern language. Rather read about architects and their practice in design. Keep track of current affairs in general and in Architecture.
• The interview can be stressful. You must stay confident. Have a clear answer about why you want to pursue a Master's degree in a specific field. 
• Group discussion will be a tiring session but don't forget to strike first. Always remember to close it with a conclusion or a way forward. Be in a steady stand and don't conflict with your statements, just weigh your words and shoot.

Visit the CEPT website and read all the info. If possible do pay a visit once before applying for the exam. Roam on the campus, talk to people there, which surely would motivate you to prepare well in an indirect way.