An addendum to this advice: generally, when I climb out of the saddle, I pedal at a lower cadence. My respiration is often coordinated with my cadence (e.g. out for three pedal rotations, in for two), and sometimes I unconsciously use the same breathing pattern when I switch to standing. If my cadence goes down and my effort goes up, even if it isn't close to my maximum, the respiration rate that worked for in-the-saddle riding may not be sufficient for out-of-the-saddle riding. So, in addition to not trying to push too hard as in the post quoted above, I also focus on increasing my rate of respiration, if necessary. These two things together have helped me to be able to climb standing for longer without going into the red.