The task of cutting large panels is relatively easy with a table saw. By the same token, making square cross-cuts on planks is a piece of cake with a miter saw or a properly tuned radial-arm saw.
The question is, what do you do if you've got a circular saw, but don't have any of these woodworking machines? Easy. Build a couple of simple woodworking jigs.
My two favorite such jigs are essentially the same jig, just in different sizes. Out a sheet of melamine, or better yet, MDF, cut a 12" x 24" rectangular piece. Then, using a straight-edge, cut a 12" long x 2" wide strip of plywood. Be certain that one side of the plywood is as straight as possible, as this will be used for a straight-edge on the jig.
Using a framing square or layout square, mark a straight pencil line six inches in from the right end of the melamine/MDF board (18-inches from the left end).
Next, using a power drill, attach the plywood strip to the board on the left side of the line. Be certain that the straightest edge of the plywood strip is tight against the line.
Once the strip is secured, place your circular saw against the right edge of the plywood strip (on the pencil line) and cut the melamine or MDF board, using the plywood strip as a straight-edge.
The jig is now complete. To use the jig, simply mark the piece of stock you wish to cut and place the jig on top, lining the freshly cut edge of the jig with the marks on the stock. Clamp the jig in place (be sure that the clamps are to the left of the straight-edge) and cut the stock in the same manner that you cut the edge off of the jig in the previous step. If you built the jig correctly, the cut on the work piece should be perfect every time.
The beauty of this jig is that it can be adapted to pretty much any size. I like to have one jig that is 12"-16" wide (for cross-cutting small boards) and another that is at least 4-feet wide (for panel cutting). Having a large version of this jig makes cross-cuts on a sheet of plywood simple and precise.
Even if you have access to a miter saw, radial-arm saw or table saw, these jigs are handy to have around, especially if you find that you are working on a project away from your shop. For instance, if you're helping your neighbor build a backyard playhouse, do you really want to haul your miter saw or go back to the shop every time you need to use your table saw? A couple of minutes to build these simple jigs will solve the problem and still give some impressive results.