Balmforth of England (who might know a thing or two about leather grips) recommend using denatured alcohol to clean leather grips, and a light rubbing of castor oil to restore tack and softness. I've used these products and they work well. Some stuff to remember:
Don't use too much alcohol, or rub the alcohol too hard - it can parch the leather and cause chunks to come off the edges of the grips (or make them stretch or shrink, or come unglued).
Don't ingest the castor oil.