The next time you need to trim off half-an-inch or less, clamp the molding to a scrap piece of board thatís wider then the molding. Like so. Now, itís important to make sure the board extends past the end of the molding.
Then use smooth, straight strokes to cut through the molding and, partially, into the scrap board. The board will provide a rigid support for the molding and help prevent the waste piece (here) from breaking off before youíve sawed all the way through.
You also need to be careful not to force the saw on the push stroke, or the blade will tend to flex and pop off the waste piece even with the board in place. Cutting off small portions of molding this way will help you get smooth, precise cuts every time.