Say you are preparing a camera-ready submission and you are running a little low on space, maybe by a few lines. At this point, hopefully you are willing to put in the time and try rewording some of your sentences. A usual way to start is to identify a paragraph with a very short final line and try rewording a sentence within so that the paragraph uses one less line.
But if you have tried this route, you may notice that TeX has “optimized away” your effort by subtly padding the pages with more vertical spaces, thereby keeping the page count constant…
Long story short, you want to use
\raggedbottom. Put it in the preamble and recompile. With this, LaTeX will keep the breaks at the same places, but it will not pad and hence the bottom of the pages will be ragged (duh!). Now you can usually see why your effort did not produce the desired effect: the space that you just freed up does not allow the current page/column to absorb enough material from the next.
With the ability to find a short page/column by inspection, now you can identify the earliest place where rewording is more likely help. Repeat the reword-recompile cycle a couple times, and the page count will go down. Just be sure to take out
\raggedbottom when you are done!
P.S. The two-page mode in your previewer can make the inspection more effective.