I use Vacu Grips. Their grips are numbered and have the fractions. The numbers have 1/2 sizes. Generally go down 1 1/2 sizes from what you used before. I like them pretty tight. Seems it only takes about 15 games and they loosen up. I think Vise recommends going down 2 whole sizes (number wise).
I use 1 1/2 sizes down and glue the grip all the way around the bottom of the grip before sliding it down in the hole. This creates a "VACuum" when you mash your fingers into the grips, the air has nowhere to go.
Also, another tip, I would be careful on this one, but I use an Xacto knife and cut the grips off flush with the ball. I leave this edge sharp most all of the time, as it creates more vacuum to me.