I agree with Naya. Of course, drugs can be used as supplements, but are generally adviced against. Or at least not relied on. I have studied shamanism, as historical system and as personal system, for years now and I can say that you can achieve completely drug like state of mind with simple methods. I personally favor the northern tradition of drumming, but I am pretty sure anything monotonic action suffices.

By overcoming your daily state of mind you can dive a lot deeper in your head. Whatever form of shamanism you take of, it is all about changing the state of mind into something most of the practices call "spirit state". It might be that all that happens in that state is just in your own head ( this doesn't support tradition, naturally ), but it is notheless very interesting experience. I doubt there will be any good way to prove scientifically that you travel to the spirit realm and such, but I think it more as personal quest for enlightement. For me that suffices.