There are two kinds of stress: distress and eustress. Stress is what you feel when you're in a situation where your fight-or-flight reaction is brought into play. Everybody's familiar with distress, the unpleasant version of stress (compare discomfort, dystopia, dismay). It's what you feel when you are in a bad situation you can't get out of. You got a bad grade and you have to show your parents. You're being hunted by a tiger. You're on a rollercoaster and it's climbing up the hill and it's too late for you to get off
Eustress is a much less well known concept, but one I like a lot. It's the positive form of stress. Compare euphoria, utopia, eudaimonia. Eustress is what you feel when, if you don't give your all to something, there could be really bad consequences (the kind that would distress you), but that you can still work hard and overcome. It's what you feel when you're pulling an all nighter to finish a project. You're hunting a tiger. The rollercoaster has gone over the top of the hill, the dread is done with, and it's nothing but exhilaration.
Eustress is, for me at least, a very reliable way of getting into a flow state. A lot of my best work has been done under eustress. Setting deadlines where there are real consequences if you don't follow through by then is the best way I know of to get yourself into a eustress mood, but I'm sure there are others. Most of the time it comes from the environment forcing you to do something
Eustress is great for productivity, but I don't think I would be able to live in that sort of a state for very long. Fortunately, since it's almost always caused by a time crunch, it's inherently temporary. And the relaxation after finishing a project and letting all that stress drain out of you is like none other.
So go out and foment some eustress in your life! Set yourself to work on projects it will be hard to complete. Give yourself a ridiculous deadline to do something really audacious. Really grab the bull by the horns, and see how far you can ride it! It's sure to be further than you think.
The one caveat to keep in mind is that eustress and distress both treat your body the same way. You probably know how stress affects you-messes with your sleep, your appetite, gives you a canker sore, makes you worse at healing, all those things. Both eustress and distress will do those, so you should make sure to give yourself time to relax and recuperate after any particularly stressful events, even if they're eustressful.
PS I'm pretty sure that eustress is supposed to be pronounced you-stress, but I really like thinking of it as oy-stress, like boisterous.