Apply Clamp Pressure in the Right Spot
Screws are a quick and sturdy way to reinforce simple joinery, but usually you'll want to hide the screw heads when you're through. There are two ways to do it: smear on some wood putty and hope it blends in well enough with the finish color of the project (it almost never does!) or cap the heads with wood plugs. The big advantage to wood plugs is that you can cut them from a scrap of the same lumber as the project, so their tone and grain blend right in. Wood plugs are a breeze to make and install. Once you try them, wood putty will seem like a poor excuse.
As shown here in the photo, instead of simply pushing the clamp pads against the legs, insert a pair of pressure blocks between the pads and the legs. These pressure blocks should be squared up and float freely between the clamping pads and the assembly that you are building. (Don't be confused by the clamping pads attached to the clamps.) The pressure blocks are directly in line with the rails and guarantee that the inside faces of the legs will seat squarely against the end grain of the rails.