Square up that miter gauge easily
Like many woodworkers, I've generally perceived my tablesaw's miter gauge as a nonprecision instrument—that is, until I discovered a simple way to accurately set it for 90° cuts. In truth, it's more difficult to describe my process than to actually do it.
Start by preparing a test piece—MDF works great—about 2' long and as wide as the maximum crosscut capacity of your saw (typically about 13" for a contractor-style saw). Make sure the long edges of the test piece are absolutely parallel by ripping them on your tablesaw.
Set the miter gauge to 90° and the blade height to just a hair over half the test-piece thickness. Mark one face of the test piece "this side up for first cut" and make the first cut. Turn off the saw and flip the piece so the formerly far edge is now against the miter gauge, and the marked face is down against the table. Mark the second face as shown and carefully align the kerf from the first cut with the blade, then make the second pass. The edge of the test piece shows whether your miter gauge needs an adjustment. If the half-kerfs diverge, make tiny adjustments to the miter gauge and repeat the test until the pair of cuts line up perfectly.
—Jonathan Leavy, Newton, Mass.