Most birds that have Sour Crop have internal canker that is blocking the crop passage. You'll want to treat it with Flagyl (metronidazole) and/or Baytril. It can be extremely hard to treat when the bird starts 'gushing' water and all the contents of the crop, because it can't pass through it. So if you squeeze it just barely, it can push the contents up through the mouth (which is dangerous in itself because the bird could aspirate).
The best way to prevent Sour Crop or any kind of crop stasis, is by keeping the birds healthy. Canker is common and if you see any signs of the birds being sick or not acting right, you'll want to pay special attention to the bird. It's easier to treat sick birds if you catch it early on. Most cases of canker occur in the mouth/throat and are easy to get rid of. All birds have trichomonads, which are what cause canker. But when they are stressed out or exposed to different strains of it, the amount of trichomonads can multiply and cause sickness. The same thing happens with e. coli and coccidia, which are also naturally present and usually don't cause problems.
A couple years ago we had a bad flood down here. I had some birds in a ground aviary, and it was down in the creek bottom. The water came all the way up and into the aviary, bringing all that gunk with it. And the food got wet. Before I could get down there the next morning to change everything out, the birds just have eaten/drank contaminated stuff, because almost all of them ended up with fungal infections. It caused crop stasis much like Sour Crop. It was horrible; killed a lot of birds. But I did manage to save a few. I gave them all an ACV 'shock treatment' - 2 tablespoons per gallon. I also gave them a 1/4 of a FishZole (metronidazole) pill daily. I only gave them access to water and food twice a day, in hopes that it would give them more time to pass the contents before filling the crop even more. If the birds didn't throw it up first, then when I picked them up, you could see the water rise to the top of their throat (in which they'd throw up water on me). It was a very depressing epidemic. I stayed out of school the day I noticed they were throwing up, so I could try to save everyone.
Keep your food and water fresh, and the dishes clean. Canker and most diseases are passed mainly through contaminated food/water.