I don't like giving secrets away, but this one is available for sale now and soon everyone will know about it. My Father made me make one of these when I was Very young and it was great. The new, modern plastic version is available from Bench Dog tool company;

Bench Dog Procut saw guide

This version of an old idea is well balanced for a wide range of applications from flat smooth 2x4 stock to wooden I beams and siding. Take a look at the link and you will see how simple it really is. It works with any circular saw with the blade on the right. Sorry helical drive guys, with your blades on the left. (although, you could also use this tool by simply hooking it on the far side of the material.) This tool has a guide rail on the bottom that holds the guide square on the material. On top is a rail that is exactly 90 degrees to the edge of the material. Just slide your saw across the guide and you will have perfect 90 degree cuts WITHOUT ugly saw base marks. Easy to set up cuts? Oh Yeah, Just measure from the end, to the edge of the guide and hold it tight. Make your cut and you're done. I'm serious, this is an old secret that is out of the bag now. Trim carpenters have used these for years to protect factory finishes on shelving, siding, laminated flooring, laminated countertops and paneling, I use a home-made 48 inch version to cut plywood and paneling square and with minimum tear out. Check out the link and you will believe.

Want to order one now?;

Amazon.com Bench Dog Pro Cut saw guide

Tom in KY, just sharing with my friends Dad,,