I saw an interesting ballasting idea on Nick Husen's Free-moN "Troy, MT" module about a month back: He laid his down using sanded grout, as you might use between bathroom tiles. My initial thought was that it looked pretty good because it seemed to have a fine grain size. IMO this is one of the biggest problems and particularly so in N, and because, being grout, the grains were filled in between with solid material right from the get-go. I thought they looked like they were sitting down, tightly interlocked, rather than having a top layer that sits up, where you can almost see light underneath the top surface grains/rocks.
The whole point of real railroad ballast is to immerse the ties on top of a solidly interlocked (read: unmoving) surface and to embed them too, so they do not shift around and let the ties move sideways. That's why the rocks are sharply pointed--so they interlock--rather than water-worn, like some decorative gravels. The latter can easily shift and move, even if you just walk on it, and the sand grains, like those suggested by Selector just above look broken and pointed, at least to my eye.
Personally, I like to fill in between the grains with a fine dust, but that grout idea is something I really do want to try. If it's easy to use, it might take one step out of the process, and it also shouldn't blow away if you sneeze on it before you get it stuck/fixed in place.
As to other fixatives: Another you might try is artists "Matte Medium," which is a goo sometimes mixed into acrylic (water based) paints, or used to give an overcoat after the original painting is finished (so the paint doesn't chip off). It comes in several finishes, Matte, Satin, Gloss, etc, but matte is what you want. You can thin it to whatever consistency you want, and apply it with an eyedropper. It looks like you just poured a thin "milk" onto your ballast when you first put it on, but, it settles in and dries clear.
That said, you can thin down white glue as well, and the glue can sometimes be softened up again with a sponge or wet rag laid on top if the need arises, while the medium will sport an invisible, but rubber like surface which is permanent once set down. I saw Tony Koester comment that he like to fix his static grass in place with white glue, so he could come back, wet the surface again, and straighten up the fibers using the static applicator tool almost as you would a handheld vacuum cleaner (go figure).
In any case, I would test any ideas/methods first on a short section of track to see if you like it, and to see if the process works for you.
I would also, as has already been suggested, add some drops of dish soap like Dawn, to thin any fixatives still further during application. I have not tried alcohol myself, so I can't comment on that.
Dish soap is a surfactant--it breaks up surface film, like the surface tension of plain water, and Dawn happens to be a brand I've seen used by animal rescue people on seabirds when there is an oil spill. The birds cannot fly because their feathers are glued together with oil, but giving them a good strong bath with soap can get them working properly again--it thins the sticky stuff right out.
Try some different ideas to see what works for you. I doubt you will find anything that's "best" for everyone, but if it suits you, that's all the best you'll need.