What is the most efficient way to advertise your site?

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I'd just like to know with which advertising methods you had the most success with(signups, refferals)? Paid to read: which services of them. Banner ads? CPC, CPM (for example AdWords). Emails (how do you avoid spamming). Signatures (in forums, email, etc)? Paid search engine top rankings? Paid directories (yahoo directory)? Classified ads (Craiglist)? Is there any other methods?