Often I'm out of training because of a lack of schools around where I'm living. But when I can, I train Brazilian jiujitsu. During those times of no combat sports training, what kind of strength, conditioning, mobility, and prehabilitation work should I do to stay ready for BJJ?
If BJJ is around, you should do BJJ. If grappling of another kind is around--wrestling, judo, SAMBO, et cetera--you should train that. But if nothing's available, you should do general physical preparation with a slight emphasis on BJJ's specific requirements.
For instance, you'd want to do some kind of general strength (not bodybuilding!) training. This keeps you able to do the basic techniques of the art, and helps injury-proof you. A balanced program generally includes:
The pulling would be of slightly higher priority than the pushing, just because A) nearly all of us tend to hunch forward because of desk jobs and B) BJJ needs more pulling strength than pushing.
That general strength work should probably include at least some neck strengthening, and a minor emphasis on grip strength. Bridging or a neck harness would be good for the former. Farmer's walks (as conditioning or a finisher), deadlift or pull-up holds, and towel pull-ups would be good for the latter. Like most athletics, one might want to do some power work, such as Olympic lifts or explosive jumps, as well.
These should not take precedence over whole-body strength. A workout might look something like this:
Notice that the sport-specific changes to a general training prescription are quite small: doing pull-ups but doing them with a focus on grip, doing power cleans sometimes, and adding a single accessory exercise. This still looks like a general program, and that's good.
You'll want to do some sort of mobility work, like yoga or stretching. Hip mobility is particularly important.
Staying in shape
You'll want to do some sort of conditioning. Being in shape for BJJ, like almost all sports, requires doing the sport itself. But the next best thing is to generally be in shape for efforts short and long. Distance running as well as HIIT are a good idea.