Try adjusting the position of the post so that the pads are closer to the caliper arm. THere is less distortion of the geometry as the arms swing if the pads are close to the arms. later on, move the pads inward (ike you now have them) to makeup for pad wear. By setting the pads like this, the arms are closer to top ow thier swing so will move up/down less, side-to side more as brakes are applied and it will be easier to keep the pads hitting the rims squarely. THis will also necessitate adjustment of the saddle cable. Shorten saddle cable for more mechanical advantage.
Definitve guide to cantilever saddle cable geometry is at;
http://www.circleacycles.com/cantile...i-geometry.pdf
and an accompanying online graphic simulator
http://www.circleacycles.com/cantilevers/