Slave Flash - Dark Picture - Limited solution

Problem:
Buy a cheap slave flash ($20 at Ritz) to complement the light produced
by camera's flash. However, you will find that the picture comes out
much darker than the picture with only camera's flash. Picture is
darker than the one with no flash at all.

Analysis:
You will find threads and articles on the Internet describing the
phenomenon. Quickly put, your digital camera flash fires two times;
once to compute the exposure settings and second time for the real
picture. Slave fires off with the first flash, camera thinks the
subject is very bright and dims down the exposure. However, when the
real picture is taken, there isn't much light at all. So, the picture
comes out dark.

Quick solution:
Buy expensive "intelligent" slave flash.

Another solution:
Check if you camera supports "1st curtain" and "2nd curtain" flash
feature. My camera (Canon S2 IS) does when in manual mode and when in
some program AE modes. When you set the camera to "2nd curtain" mode,
camera flash fires at the end of the aperture opening. When you click
the button, aperture opens, stays open to gather light, flash fires,
and aperture closes. Since the aperture is open, it doesn't matter
when the slave fires. This worked fine and the picture came out bright
and I can clearly see the effect of the slave flash. Now, I have to
figure out how far to keep the slave flash so picture doesn't come out
too bright. I can live with that.