using online jobsearch effectively

I have created online accounts with a few well know online job sites and configured their so called "job-agents" to automatically search and email me for matching job positions. But after a while, I realized that the "job-agents" are either not effective or not accurate in hunting jobs for you. The problem could be due to our errors in configuring the "job-agents" or due to inherrent bugs in the "job-agents". Sometimes the jobs are simply not advertised on these popular job-sites but posted on some other niche sites instead. Or there could be other reasons beyond my imagination. So, in order not to miss any job opportunities, you still have to actively visit the websites and manually search for jobs. And to be a typical "kiasu" catch-all, you can use the simple yet powerful search engines such as google or yahoo to search for jobs, you never know what you will stumble upon as long as you have the time to go through the returned results.