Two triangular maple blocks and a pair of sharp-tipped drywall screws are all you need to make corner clamp blocks for picture frames. Drill and counterbore a hole for a drywall screw through one leg of each block, positioned so the screw’s tip is centered in the hypotenuse (photo). Make sure the screw threads through the shank hole, so you can turn it in and out. For hard woods, extend the screws so their tips barely protrude; extend the tips a bit further for soft woods.
To use the blocks, assemble the joint, with one frame side clamped to your bench. Press a clamp block to each side. When you clamp across the joint, the screw tips bite in and keep the blocks from sliding. The screws leave small dimples, which can be easily taken care of with wood filler or a few drops of water to raise the grain. To accommodate joint angles other than 45°, simply modify the shape of the blocks.