I really recommend The World Of Shamanism by Peter Walsh, it has really opened up alot for me in terms of having some clarity and perspective about the practice of shamanism.
Also, one thing to keep in mind, that alot of others have said, is that shamanism as a healing art consists of the practitioner having a direct experience with God/Nature/Guides etc, with their own inner resources to guide them from within. Having teachers to guide you I feel is also important, because the shaman is the axis mundi between the world of spirt and earth, which is very powerful yet can be quite treacherous too. I certainly have some "psychic" scars from travelling the axis mundi unsupervised. Experience is knowledge, but I wouldn't recommend walking into the worlds of ether without truly feeling safe and secure in body and mind. I have also witnessed a Reiki healing where the receiver was suffering from a very powerful form of posession and required my teacher to step in and release her from this entity. The recipient said that she had done something that sounded like experimentation with a shaman, however, I doubt that this person was legitimate in the sense that they knew what they were doing. My Reiki teacher vehemently was opposed to shamanic practice because of such occurences, but knowledge is power.
Your own intuition should always provide you with the right resources. Be discerning if you are seeking teachers. The word shaman is thrown around so easily today because there really is so little understanding of what this practice really is about, and that is partly because
there is still more education that needs to be presented on the subject. There are alot of people who take advantage of this and end up manipulating others who seek advice and training. I would say especially be wary of anything online that promotes itself as a shamanic training workshop, because you don't know who is really teaching you or what their resources are. I would also recommend The Institute for Shamanic Studies website. You may be able to get in touch with one of the teachers or the founder there, and that seems like a really good resource.
I also have two friends from tribe and myspace who are both practising shamans. One is Native and the other is white, both seem to have alot to offer. Filip Klein, who is Native Alaskan, is a practicing shamanic healer, and Richard Gross is a shamanic and sound healer. Richard has gone to Peru and apparently studied differenty shamanic practices, while Filip has seemed to have experienced direct training as a Native American. I am still getting to know both of these people, but I feel that they are worthwhile in terms of knowledge and resources. I hope to be able to meet and work with them at some point.
I myself do not practice, but am gathering years of internal experiences combined with reading and outside resources to have a balanced perspective on this path before I make any decisions for training. I feel that balancing the inside and the outside is the most well rounded path that someone can pursue.
And of course, if you just align yourself with positive energies and power, and not give any attention to things that do not serve you, that always opens the best doors. Ask the universe for what you want, the universe will always give you exactly what you need. Best of luck, and love and light to you.

myspace and tribe: Filip Klein

tribe: Richard Grossman