keep (one's) powder dry
To be prepared to act with little advance notice. The phrase refers to gunpowder, which needs to be kept dry in order to work properly. I know this job opportunity didn't work out, but keep your powder dry for the next one that comes along.
keep one's powder dry
Stay alert, be careful, as in Go ahead and take on the opposition, but keep your powder dry. This colloquial expression, which originally alluded to keeping gunpowder dry so that it would ignite, has been used figuratively since the 1800s but today is less common than take care.