Combined Defense Service Exam is conducted by UPSC twice in a year to select candidate for service selection board’s interview for commissioned officers in army, navy and Indian air force. Following are some tips which will help you in cracking the examination:

Avoid cramming rather understand
Solve as much previous year papers as you can
Plan and study well
Study each subject specifically
Consult a good study material
Revise more

The CDS examination generally covers following topics:

General English
General Awareness
Verbal Ability
Quantitative Aptitude
Reasoning etc