For all but very high-level positions, this strategy is the most potent: Send a brief but compelling email to 50 potential employers who are NOT advertising a job you're interested in but who have the power to hire you for a position you'd accept. Follow up with a phone call one day later. For lower-level jobs, the most potent variation on this theme is to simply walk in and pitch the employer.
Why is that strategy so potent?
A job opening is born when an employer has a need but doesn't realize it. If the employer receives your email/phone call or drop-in visit at that point and you help him realize he has an important unmet need that you could fill, he might hire you then and there, without your having to compete with a zillion applicants.
If the employer receives your email when she realizes there's a need but hasn't had time to fill it, you could be perceived as manna from heaven and again get hired with minimal competition.
If the employer receives your email when he's just told insiders about the job, again, your competition is not enormous.
Disproportionately, job openings get advertised when they require rare skills or are so bad that no insider wants to take it or recommend it to their friends. In either case, such jobs are less likely to be better for you than the jobs you could be offered if you contact the employer earlier in a job opening's lifespan.