You're right that managers who understand your job can make life a lot easier. But this goes both ways. I find that understanding what constraints my management is under can help me to help them plan.
When you understand what their problem is (e.g. we promised X to the client and we have to make it work somehow), you might be able to rearrange things to make that possible. Of course, an ounce of prevention is worth a pound of cure: if you can help them avoid promising the impossible, you'll be a lot better off. Otherwise, you end up suffering due to arbitrary deadlines created for the sake of looking good. If you can find better ways for your boss to look good than impossible deadlines, you'll be way ahead. I grant, though, much of this is easier said than done.
When you understand what their problem is (e.g. we promised X to the client and we have to make it work somehow), you might be able to rearrange things to make that possible. Of course, an ounce of prevention is worth a pound of cure: if you can help them avoid promising the impossible, you'll be a lot better off. Otherwise, you end up suffering due to arbitrary deadlines created for the sake of looking good. If you can find better ways for your boss to look good than impossible deadlines, you'll be way ahead. I grant, though, much of this is easier said than done.