How do I keep homemade gelato form freezing too hard?

Adding alcohol to ice cream/gelato definitely effects how hard it freezes. From a NYT article:

'Adding alcohol, whether zinfandel or rum, to ice cream enhances both flavor and texture. Alcohol lowers the freezing point, so ice creams stay creamier and smoother because they do not freeze as solid and are less likely to form ice crystals.

Too much alcohol can prevent ice cream from freezing at all. A rule of thumb is to add one and a half tablespoons of 80-proof alcohol for every quart of ice cream base. Experiment with a plain vanilla base till you get the flavor you like; a dark rum will have more flavor than a light rum, for instance.'

I can confirm from personal experience that too liberal a hand while adding the alcohol will result in Zuppa di Gelato.

This is really tough and i don't have much luck either. Agree with chez_mere that you could increase the fat content, but I don't think adding alcohol will help much. I once heard a radio program about ice cream and the guest was explaining that store-bought ice cream stays soft because of the amount of air in it. The fat particles are broken up by whipping and they hold pockets of air (or something like that). Maybe try whipping the ingredients a bit before freezing?

Depending on what sort of flavors you are using, you could try adding a little alcohol. Maybe kaluah with a coffee gelato, or irish cream with chocolate gelato. While not traditional, the alcohol will prevent the base from freezing as hard. Another solution might be to increase the fat content, but again, this would be a little uncharacteristic of gelato.