Karl,
I have tried two of the ready-made solutions commonly available. The first is the stuff carried by Rockler. It is a thick milky liquid that you cut with water. The second is the Super Soft II sold by VeneerSupplies.com (part of the Joe Woodworker site that Tom mentioned). It is a clear liquid that comes in a squirt bottle.
Both have their pros and cons. The Rockler treatment has to be wiped or brushed on (or submerge your veneer in a bath of the stuff) and then sandwich the sheets between fiberglass screen and newspaper, changing the paper as it becomes saturated. I've had good results with it.
The treatment from Veneersupplies (http://www.joewoodworker.com/veneering/flattening.htm) is easily applied, but they do not suggest you use the screen. I ended up with the mirror image of the daily news on my veneer and was none too impressed. I may try it again, but will use screen next time. The manufacturer also claims you can use an iron to speed the dry time of the softener which would omit the need to dry it between newspaper.
Regardless which method you use, store your veneer clamped between two panels. It may need to be re-treated if you do not use it right away.
