Dish
Provide your rabbit with a dish instead of a bottle. Rabbits might drink more water from a dish than from a water bottle, because it's more natural for them to drink that way. Use a heavy dish such as a ceramic one so it won't tip over easily. Change the water every day and wash the dish with soap and hot water to keep bacteria from growing.
Fruit Juice
Ask your vet if you can add some sugar-free fruit juice to your rabbit's water. A tablespoon or two of juice might be enough to encourage it to drink. Try apple, orange, cranberry or carrot juice.
Syringe
If you need to use a syringe to give your rabbit fluids, add a small amount of fruit juice to the water or dip the tip of the syringe in honey. This will encourage your rabbit to take the syringe.
Wet Veggies
Provide your rabbit with wet vegetables to increase the amount of liquid it takes in.