I started using 100% vegetable glycerin a week ago and love it. I had a tough time finding it, though; none of the local pharmacies had it, so I had to buy it at drugstore.com. The amount I add depends on the brand of soap or cream that I use.
I melted my regular shave soap in the microwave, added a teaspoon of glycerin, mixed, then pressed the soap back into the mug. It now provides me with a better, slicker lather. Although, I think that I could probably use another teaspoon in there.
For cheap creams, like Arko, I use 4 drops of glycerin in the bowl before lathering. The lather comes out much better.