On Fri, 19 Sep 2008 00:47:05 -0700 (PDT), wrote:

in the past I've tried things like drilling a 10mm hole in the corner so
that I could start the jigsaw in there. I've never got a good result
though.

That is what I would do but ensure that the edges of the hole were a few
mm away from the final cut positions. Cut down the board and past the hole
to the crossing line. Back the saw past the hole and cut into it, repeat
the other side. Then join the two holes. Finally start on the cross cut
near one of the holes and trim the cross cut as required to the meet the
other down the board cut and then reverse for the other corner.

--
Cheers
Dave.