When I’m making picture frames, I like to “beef up” the mitered corners of the frame with a spline. As you can see in the photo, using a contrasting wood provides an opportunity to highlight the joint. Or you can make it nearly invisible by using the same type of wood. Either way, the spline adds a lot of glue surface that really strengthens the joint.
To cut the kerf for the spline, I use a shop-made jig that straddles the rip fence on the table saw, as shown in the drawing below.
Screwing two supports to the face of the jig at a 45° angle to the blade forms a “cradle” that holds the glued-up frame securely in place. All you need to do to cut the kerf is set the frame in place and make a pass across the saw blade. (I use a rip blade to cut a flat-bottomed kerf.)
Once the kerf is cut, you can glue in the spline. After gluing and clamping the joint (see detail), remove the excess material with a handsaw and then sand the spline flush.