A friend of mine swears that if you're making say a gin & tonic, filling the glass with ice will result in less ice melting and therefore your drink will be colder but less watery.
1. Prepare a glass with a little ice
2. Prepare a glass with a lot of ice
3. Pour the same amount of cocktail into each glass (make sure it's identical)
4. Wait
5. Simultaneously strain both glasses into measuring cups
6. See how much liquid came out of each glass