Here's a summary of what I've discovered so far about the progression of meditation in various traditions. At the end is an idea of my own.
1) Shamatha with object
Sometimes this is done with a physical object (e.g., a pebble). Usually, it is done with the sensations of breathing. This is true in Theravada (anapanasati), Mahayana (Zen has "susokukan" as the introductory zazen), and Vajrayana has it too.
In Theravada, anapanasati alone is considered enough to attain enlightenment (from Wikipedia).
2) Shamatha without object, "with attributes"
This is variously called "shamatha without a sign," "unsupported shamatha," "awareness of awareness" etc. One rests one's mind without any particular support. This can be practiced in informal settings to some extent, for example while walking one might think "I am maintaining presence of mind." As such, there is still a subtle clinging to a meditative state, and a definite subject-object duality (I am meditating on something).
3) Shamatha without attributes
Sometimes called "released shiné" (by Dzogchen Master Namkhai Norbu Rinpoche) or simply "resting in the natural state." Similar to the previous, except there's no longer an explicit sense of resting on or in anything.
In Dzogchen, this is sometimes accomplished by first resting in shamatha (Tib. shiné), then investigating into "who is resting," and then releasing. For some time, one is able to rest in rigpa (which has already been pointed out by the master in the Direct Introduction, which is essential in Dzogchen).
In Mahamudra, the Direct Introduction / Pointing Out Instruction is optional. Alternatively (per Lama Gendun Rinpoche), one may simply practice by releasing all fixations (including the sense of meditating or a meditator) and rest in "the natural state." As one progressively relaxes deeper, one comes to know one's true face. Another approach (Thrangu Rinpoche, Vivid Awareness) is to do analytical vipashyana to determine that the essence of mind is a cognizant emptiness (that's the "who" that's resting), and then just rest in that. The key point is to abandon all hope and fear, all idea of something to get and someone to get it.
In this way, one practices "undistracted nonmeditation," where one's self-aware presence of mind simply sustains itself for some time. Over time, it starts occurring spontaneously while off-cushion, and eventually the barrier between meditation and post-meditation blurs more and more. At its culmination, one is spontaneously (non)meditating all the time, and there is no longer any sense of a subject as distinct from objects. This is the first realization of emptiness of phenomena, which marks the dawning of the first Bodhisattva bhumi.
How does this happen? Basically, all distraction is distraction from awareness. Since awareness is the (non)thing doing the "watching," this means that distraction is awareness distracted from itself. It is characterized by determining phenomena to be external to awareness, and then awareness entering into a dualistic relationship with them. This is what gives birth to the ego, and is known as grasping. As it becomes clearer that there is nothing apart from awareness itself, there is no longer anything to grasp, and hence no possibility of distraction.
This practice seems similar to Zen's shikantaza ("just sitting"), although without careful guidance one runs the strong risk of meditating (stage 2). This is also known to be a major stumbling block for those trying to practice Mahamudra / Dzogchen without guidance.
4) Sky-gazing
Sometimes called "mingling the threefold sky" or "namkha arted." This is an important Dzogchen practice to enhance one's released shiné. Basically, one mingles one's consciousness with the infinitude of the sky, thereby actively undoing the subject-object duality.
Here's where I try something of my own invention. In sky-gazing, one has to have a clear and unobstructed view of the sky. Even clouds are obstructions. Since that's not easy, I'm considering experimenting with the
Ganzfeld Effect, where one's visual field is immersed in a white haze. One site describes the effect as "
Ganzfeld creates the illusion of an open field of vision of infinite depth." Bingo! Typically this leads to hallucinations, but I suspect most people aren't practicing released shiné while wearing them...
So I'm going to build my own Ganzfeld Goggles (swim goggles augmented with glass frosting spray) and see what happens.