I knew a guy with real Rasta dreads. The process by which it is done is as basically, you stop washing it, as others have said earlier, for a couple of weeks. The hair will naturally collect into clumps and become matted and greasy. At this point you need to twirl the clumps around and around with the fingers as much as you can be bothered. By continuing not to wash the hair, the scalp will get used to it and stop producing sebaceous oil and become drier, but you must continue to twist the mattes everyday.
He was very proud to tell me, once his dreads had reached 1.5 ft in length that he had not washed his hair in 3 years. If you can live with that (and also your family members) then this is the real method. There's no denying it though, for a peroid at least, you will smell terrible.