Bob, and all those wanting to saw "straight" ...
Some might argue it is about developing a sense of plumb. To some extent this is correct, but then what about dovetail angles? I have read advice to angle the board so that the line is plumb. I consider that this misses the mark. Even Rob Cosman makes a big deal out of squaring the board to be sawn. I prefer that the board is square as well, but this will not help you in the important area.
What is the secret? Actually, it is quite simple, and it goes for tenon cheeks and shoulders as well. In fact, it goes for ALL sawing.
When we mark out Tails or cheeks (or whatever), we draw the line across the top and then down the sides.
You will always saw straight if you saw two lines at the same time! Saw the top of the tail board lightly, and run the teeth over the side to the following line.
First this ...
.. then drop the saw ...
Curt's recommendation of the Kerf Starter is a good one (photos below from a review I wrote some years ago). I just score the line (or use a pencil), but eventually you will develop a sense of "square".
This can really help start a saw and reduce the concern that forces you to watch the top line and not both lines simultaneously.
Regards from Perth
Derek