LaTeX may not be the easiest tool for this, as in my experience notesheets cram the most info when fitting sections together like a puzzle. Using columns and arranging simply from top to bottom creates extraneous whitepsace to the right of short lines of text.
LaTeX is made to use lots of whitespace for good typesetting, whereas you're in a custom layout that minimizes spacing. Though the
savetrees package mentioned by Sean Allred will minimize some whitespace, you'll notice on the last page of their example document that to the right of the short lines of poetry near the bottom, there is a lot of space where more information could be easily added in a layout program like Scribus, but not so easily with LaTeX.
Using the free software Scribus, I would use its LaTeX Render frames to typeset each section of mathematics, and use Scribus drag-and-drop interface to arrange the text across the page to squeeze it all in. As a plus, this makes it really easy to draw lines between sections to help with visual grouping.
If you decide to use this method, when exporting as a PDF you can safely ignore the errors, and make sure to select "Embed PDF & EPS files (EXPERIMENTAL)" so the LaTeX is exported as text, and not a rasterized image. Using this method, I quickly created the example layout below. Note that the LaTeX text boxes are actually overlapping in some places, but it is still fully readable.