I'm not saying there is no possibility, but it is my conviction that the experiences of a spirit realm to date more closely resemble the exploration of the interior of the mind than of some exterior world of spiritual beings. That said, this by no means degrades their meaning, if anything it infuses them with deeper meaning. For these realms exist within are, and are part of us, and can be discovered and explored. Within my own spirituality, the concept of exploring spiritual worlds, especially through the aid of entheogens, is rich and central, so by no means do I intend to suggest that these planes are somehow unreal or faked. It is just that their nature is not as it might initially be supposed.
Here's my theory for how they do it: you must first really, really want to believe it, cast aside your logical reasoning, meditate, do drugs, have a weird dream, or whatever, then mentally distort that totally ordinary experience until you believe you have reached the spirit realm.
Almost. To be thorough and scientific, I must acknowledge that we are all far too ignorant to declare something is definitely impossible, so long as it's not contradictory. However, the "spirit realm" goes in the same mental-file where I keep deities, ghosts, fairies, and psychic powers: so absurdly unlikely as to not warrant any serious consideration.
For me (sunni Muslim), one feels the 'spirit-realm' when they're able to feel the beauty in existence, in spite of the suffering that exists...an acceptance of life being as it is (with its pleasure and pain), with a hopefulness - I'd say things like human will, our ability to rise above circumstances, and create something lasting are all a part of it- its also love/self-sacrifice. It's sort of the opposite of whimsical self-indulgence, and selfishness. (Granted at this point, there are plenty that argue that everything is selfishness, and self-sacrifice is nothing but wishful thinking. )
Guides can help, and one can enter trance like states when meditating.
Christians have a constant connection with this realm through the Holy Spirit. Whether they choose to use it is their choice. In my experience, the spiritual world is interwoven into the physical, and can directly influence it.
I'm not entirely sure what you mean by a "trance-like state". Sometimes people are unresponsive, usually when God takes them somewhere to show them something (usually called a vision), and sometimes they simply see and interact with both at the same time, something like one overlapping the other. Sometimes it's in dreams. There really isn't any set way for Christians to enter this realm, nor is there any set way for Christians to utilize it for God's purpose (unless otherwise specified by God.) The only requirement is spending time developing one's personal relationship with God.
It sort of overlaps ours, from what my 'friends on the other side' tell me. Some things(such as items used in magic) they can interact with more easily than others, as those items are already closer to that realm.
I've also heard some animals exist partway between this world in the next. Cats, for example, are said to be able to walk in and out of a magic circle without breaking it.
Though I'm aware that dying is only one method. One that I, presumably, can't turn back from. One way ticket. Others claim to use meditation and the like to make contact with the spirit realm, but if I could do does like they're doing, I'd have done. Though it might be that results vary from person to person. I can't say for sure.