Mix fresh yogurt with water (1 part yogurt and 3-4 parts water), ideally with a blender. For sweet lassi add some ground cardamom, cinnamon, ginger and cane sugar.
For the salty variation (sometimes called digestive lassi) use a pinch of cumin, coriander, turmeric, ginger, and salt.
Ginger water: 2-3 slices of fresh ginger root in 0.2 litres of hot water that has been boiled for at least 10 minutes. You can keep this ginger water in a thermos flask, and drink portions of it throughout the day.