Jeff Snell has all-but-solved the problem of how to keep your saw blade perfectly aligned when hand-cutting joints. He invented the AngleMag, which uses four neodymium magnets housed in a super-low-friction glide to hold any backless saw, such as a Japanese pull saw, at an exact cutting angle.
The solid cast aluminum body of the AngleMag clamps to workpieces up to 2″ thick with three clamping screws. The pivoting guide rotates to any angle from 45° to 90°. The guide rides on a shaft with grooves spaced every 90°, so you can lock the guide for angles either in the horizontal or vertical plane. After cutting the first angle, you can rotate the glide shaft exactly 180° to cut the opposing angle — very useful for cutting dovetail joints.
Although the AngleMag retails for $140, you can get it for about $100 from either of the online retailers below.