Very interesting question.
Having long been fascinated by body language and micro expressions, I incorporated that into my training. This won't even remotely be an option for most people as the time it took to even develop a functional use when not under pressure was well over a decade.
That said, a few tips that came from that training that have been of use to others:
Shift your focus: In his nearly complete rubbish book PhotoReading, Paul R. Scheele talks about looking straight ahead whilst imagining an apple or similarly fist-sized fruit at the top rear of your skull, imagining the weight and heft of it. Now, without moving your eyes, shift your focus from the bridge of your nose to that point, allowing your eyes to get a bit wider and take in more information. It's garbage for reading, but I find it immensely helpful for seeing more of my opponent.
Think about what you're doing: Becoming aware of what you're doing and how you project those movements, even microscopically, can help you recognize them in others. The most detrimental thought most of us have to our own survival is that we are unique when in fact we are just like everyone else. We have the same tells, the same cues, and we give them away the same ways. If we understand ourselves, we can better understand our opponents.
Play guessing games: Silly, I know... But make a guessing game out of sucker punching. You're already controlling one aspect (namely that he is going to throw a punch at you); now guess at the other aspect. Have him think about where he's going to strike and watch how his body shifts. A nifty little trick? When a person thinks about one hand too much, their nose will point toward that hand. Is this significant to striking? Not necessarily, but it's about learning to look for subtle clues that may help you later.
Keep the big picture in mind: If you focus on any one individual aspect, you'll lose sight of the whole. Like the Thousand-Armed Kannon (PDF, page 5) (千手観音 Senju-Kannon in Japanese), if you relax and take in everything, everything is in harmony and all your arms can do what they need to do; the second you focus on one aspect, everything else becomes useless.
This probably isn't exactly the sort of answer you're looking for – there's a lot of non-martial-arts-derived thinking to my methods, and some of it is pretty hard to get to understand since it's only a small part of a much larger picture. Others trained in a different art than they do now for many years, shaping how they now perform the art they're learning; I spent nearly 15 years learning to appear to read minds by simply watching and trying to understand the things that make people so similar, and it's had its own effect. I hope this can be of some use to you.
Edit: I want to make a comment about the approach you all are taking, and try to offer an alternative from my point of view which seems quite different than 90% of martial artists I've encountered...
Drilling implies that you act a certain way during the period of the exercise, then leave that behind when you stop performing that exercise. This is, of course, one way to do things, but I personally find this to be an extremely inefficient method of learning.
Consider a memory exercise: If asked to remember 20 objects in order, most people will remember between 5 and 9, less than 50%. The reasons are that 1.) we seem to have difficulty remembering unrelated objects, and 2.) that even if the objects were related in a general sense, they may not be noteworthy enough (as in the case of 20 digits) for us to have an interest to remember them. Enter mnemonics, and the anchoring of ideas.
Most of us can remember the layout of our homes explicitly (though not necessarily every item of furnishing) because they're a part of our everyday lives. As such, in mnemonics, the concept of the memory palace is to tie in things we need to remember to the everyday. My childhood home (in mental representation, of course), for instance, holds in the living room (in which I was only ever allowed on holidays, which made it deliciously memorable each time I snuck into it) representations of each of the 52 cards in a deck of playing cards. Let's just say my time in Las Vegas was rather poorly spent. Because I relate this to something "everyday" it's quite easy to remember, and I can recall the cards that I've seen at a glance by simply mentally looking about a room.
We can anchor all sorts of concepts: objects, doctrine, ideas, and actions. By anchoring actions to everyday actions (that is, by anchoring the idea that when I walk, I must defocus my vision), we can make an act natural, and thus remove the necessity for it to become learned through rote repetition (drilling). We're awake for some 16 hours of the day, we train for an average of 2 hours a few times a week. What are we doing for those other 14 hours? If we're not making our learned behaviors natural, we're not making effective use of our time.
Edit: Proposed Drills
This might sound a bit off, and since you're intending it as a method of instructing others in it, this might help: take a head band and a small piece of fist-sized fruit (or a ball, or whatever. Something good sized with some weight) and tie it around the head of one of the students. Tell them to focus on it (which, of course, is impossible... It's behind you, and completely out of your field of vision). Like the visualization exercise, instruct them how to notice that they have the piece of fruit there without looking at it; to feel it, and, by virtue of feeling it, focus on it. The game is simple – the person without the fruit must try to strike the person with the fruit in such a way as to be able to retrieve it; the other must maintain the fruit. Switch after a few minutes or a de-fruiting.
I hadn't really considered this because it's something we train for a different reason (sakkijutsu – detecting and projection of murderous intent), but it's really the same thing in a different take, but it may be one of those things you just don't understand until you're taught it. This drill is for pairs. One person (
A) stands in front of another (
B), turned away from their partner, with their eyes closed.
B holds up both hands about 6 inches from
A's shoulder blades then hits
A in the back with either hand at regular intervals. When
A is comfortable and relaxed and feeling the flow, as soon as he starts to detect a strike coming, he should instinctively move that side out of the way. This is a game in dual-suggestion: 1.)
B is suggesting to
A the strike through subconscious signals and 2.)
A is suggesting to
B where to strike strike via subconscious signals.