Summary:
In the last coupla days I've made
a simple gadget and greatly improved my saws by its use.
http://www.geocities.com/plybench/tmp.html

Long Version:
I've been trying to sharpen my own saws for a while
now. I rapidly got to the point where I could sharpen
saws better than my local hardware store (which says
more about them than me :-( ) and have been pretty
successful on large rips, dovetails, and large crosscuts.

But I've been miserably unsuccessful on small cross
cuts with teeth smaller than 8 TPI (typically
backsaws)

In his wonderful (it's my primary source)
write up on saw filing:
(
http://www.vintagesaws.com/library/primer/sharp.html
on his site
http://www.vintagesaws.com
)
Pete Taran says:
"With even a small change in the way you move
your wrist, you can increase or decrease the
angle by as much as 10 degrees. In saw filing,
this is a huge and unacceptable variation."

It is indeed very difficult to eyeball the angle
of a small 3 sided file to fine tolerances
(i.e. a coupla' degrees)

Pete suggests making a "Rake Alignment Jig"
which is a simple block, crammed over the
end of the file. When I tried this, I found
3 problems.

(1) I lost an inch off the end of my 4" file, where the
     block fitted

(2) When filing with a fleam angle, the block fouls
     the filing stroke even more so than in (1)

(3) It's cumbersome at best.

Robert Wearing ("a gadget for every task") suggests
making a "needle indicator" that slips over the ferrule
of the file handle, but his design involved more metal
work than I wanted to do, and called for materials I didn't
have to hand.

So I cobbled together a wooden version, where the
only metal was a #10x1" round headed screw.
http://www.geocities.com/plybench/tmp.html

It's just a scrap of mahogany, drilled (with a brace, natch)
to fit over my 1/2" (bit of dowel) handle, kerfed under the hole, and
with a screw to tighten it around the handle. The angle
is indicated and magnified by the bamboo kebab skewer.

I used my trusty Nobex mitre saw to make a gauge
block with 12 degree faces at each end from a piece
of lath.

To set the gadget, I just reference one of the faces of the
file on the gauge block, turn the gadget on the axis
of the handle until the indicator is vertical, and tighten.
To maintain the rake angle is now the same task
as maintaining the indicator to vertical.

I used this gadget to perform a "top and refile" pass
over my practice saw. The results were an astonishing
improvement in teeth evenness.

Now I just have to recut/refile all my fine toothed
crosscut saws...

I'll probably make a tidier version of the gadget too,
although prototypes that work well tend to have surprisingly long
lives :-)

     BugBear (with a lot of filing and better saws in his future)