Hey Terry,
I too had the same question months ago...I had several suggestions from those making sheaths longer than I have. One guy uses acetone to wet the leather, as it evaporates quick....terrible choice! It was way too strong, removing all the natural oils from woods such as rosewood, cocobolo, and ironwood...not to mention that it will also remove leather dye, and leather cement...not good. Also tried was alcohol...as it also evaporates quick, albeit, not as quick as acetone. Again, bad choice, but not as bad as acetone, but the same bad results basically as acetone, but not to the same degree. I found that both would "streak" untreated leather, even after it was dry...I dunno...just bad all the way around.
I eventually learned my lesson and went back to plain old water and resigned myself to the fact that I would just have to wait longer for the leather to dry and plan for that accordingly. Also, no dissolved leather cement, no streaks, nothing.....obviously very mild on the leather. I made sure to use hot water (as hot as I could stand) and it softened the leather better for me. And when it dries out, it the leather will be hardened and will take a coat or two of some sort of finish to complete the process...I like a lacquer based product from Tandy leather, but just plain old spray lacquer (satin finish!) works great for me. Use light coats of the spray or you will not get a pretty finish.
My two cents worth...best of luck!
Hank Hammond