Hmm... strength training for the eyeballs? I probably need this... My vision seems to get worse by the month.
It's not necessarily strength training, sometimes it's the opposite: relaxation. For me, the key was having noticed, from time to time, that when I was particularly relaxed, my distance vision was better. I don't want to even begin to talk tech about eyes here - they say a little knowledge can be a dangerous thing, and that's what I have about eyes, a little more knowledge than the average Joe or Jane. I will add one other point - as with strength training, a good deal of progress exists in the realm of the neurological.
I can give another personal example. There is a tendency, as we age, for people to start to favor one eye for reading and the other eye for distance. For some people, this happens quite naturally, they hardly notice, and they might end up with glasses that help them see like this, with essentially a distance vision correction lens for one eye and a near vision correction lens for the other. One can fight this tendency by simply being aware of it and doing simple things like, from time to time, making sure to look only through one eye, then only the other, then both and notice how they combine, and trying to make sure you're actually using both eyes for both close and distance seeing. This is something I practice and it has helped me.
My distance vision "aha" moment came when I was wearing my contact lens and had been using +1 reading glasses at a restaurant to look at the menu. I forgot to take them off, started driving, and noticed I saw better at distance with them than without them. Sure enough, I went to the eye doctor and found my distance vision had improved by 1 diopter in one eye and .75 diopter in the other, almost exactly, albeit coincidentally, what my reading glasses were.
Please don't assume any of this applies to anyone - best to do some investigating on your own. But I suspect that, a lot like the Buteyko breathing that I teach, there won't be much of a commercial future in the kind of vision correction described above because it would take business away from people who practice what are now the traditional, accepted ways to deal with these things.
Just my opinion, your mileage may vary.
-S-