Hey Bobatnathrop
The camera you are looking at has 4 flash modes. They are:
Auto, On, Red-eye reduction, Off
Here are some setup tips that work for me.
1. Use a lens with a manual aperture.
2. Be sure you have focus set to manual and not to auto.
3. Use the "ON" setting on the flash options not auto.
In tip 1, I have found in using auto aperture lens that the delay is to long and the light from the slave does not show up (yes I have lowered my shutter speed to compensate but to no avail-even at 1+second) Use a lens with a manual aperture ring.
In tip 2, leaving the focus set to auto indicates that the camera has to do its own focusing. To do that it needs light (guess where the light comes from?....the flash) So yeah turn off auto focus. Its right there near the front of the camera and easy to do.
In tip 3, using the flash setting "ON" is pretty much general and works well for pretty much everything. It will not pre-flash. That is only for red-eye reduction and for focusing as mentioned above.
Also remember to attach a small piece of developed yet unexposed film over the camera flash as you don't want light in your pic coming from that flash yet it will still set off the slaves with the IR that passes through the film.
Another tip is run the ISO as low as you can get it (ISO 100 on this camera)
When using a fully manual capable DSLR such as the one you are eying, many of the flash problems associated with many of the point and shoots don't carry over. This is true most of the time.
Below is a link to the specs on the camera you are looking at
Good luck and happy shooting!
http://www.dpreview.com/reviews/specs/Canon/canon_eos10d.asp
never stop imagining what could someday come to pass...