There are a variety of options. Most photo editing software has multiple options for adjusting contrast and getting pure B & W images. If you haven't already locked in on this class of software GIMP is a good option, and free. There will be tools to adjust the histogram, thresholding tools and tools to adjust the gamma. Also noise filters, filters to remove the silkscreen pattern used when the magazine was printed and others. Each image will benefit from different combinations of these techniques.
Don't ignore smoothing the image. Pure thresholding can leave jagged edges on lines and holes in the middle of dark areas. Upping the pixel size of the image, and running various filters (I find a Gaussian filter possibly the most generally useful) before thresholding and reducing to two colors can generate smoother traces.
Also, a vector editing program such as Inkscape (also free) can be useful. They have tracing tools that attempt to find and follow edges, which results in clean lines, not grading from black through white along the edges. There are tutorials in this forum on how to trace PWBs using Inkscape, and many of the techniques would work fine for cleaning up magazine images of PWBs.
Finally, be sure to check the geometry, including both scale and warping. For older PWBs that are mostly Rs, Cs, Ls and transistors or fets this won't be too important, but the publishers editing, printing and distributing process, followed by scanning and editing can easily result in pad spacings that don't fit large components like CPUs and connectors. The geometry errors are relatively easy to correct by scaling using the known sizes of these large components. Warps and twists are a little tougher, but once you are thoroughly familiar with the editing software of your choice it isn't terribly hard.