read the advertisement very carefully and note answers to the following questions
what sort of person are they looking for?
is the job to work
-with a specific person, with a specific group, or to work independently?
- on a specific project?
- in a specific research area?
is the job supported by a specific grant or program?
After you have gleaned answers to these questions answer:
Do I want this specific job?
Would should they be specifically interested in me?
Then if you apply make sure your cover letter specifically tells them why you want THIS specific job and why they should interview specifically YOU.