I use a product called Corral, and it definitely helps, but it is easy to get too much out of the squeeze bottle (one or two ounces per 100 gallons), and it will plug screens rather quickly.  I do some backpack spraying around vegetable crops, and it makes all the difference in the world with that.  Some small vegetable plants will just about die if you open the Roundup jug at the edge of the field, and this seems to keep the mist under control.