Conducting an online job search is somewhat like looking for a needle in a hay stack. You have to always keep in mind that the www is a giant world-wide library with no card catalog. Search engines and topical search tools can help to a degree. But in the end your success will depend on your persistance.
The key to successful job searches using the internet really start with effective uses of key words. Make sure you know what the current job titles are within a field and commonly used phrases within vacancy announcements. If you are a recent graduate, be aware that terminology within a specific resource occupation may be older than your knowledge.
Talk to currently employed resources staff and discuss how they advertise their job announcements. This may help you narrow your search objectives.