The best way to freeze your waterbottles is to fill them with water, leaving about an inch at the top (depends on what sort of mouth the bottle has) for the ice to expand. Also, if you put the lid on, do not screw it on tightly, but leave it just resting on top. This allows the air to escape as the ice expands, and should eliminate cracking issues. disneysteve's suggestion of filling partway, freezing, then adding water is also a good idea. If you have a bunch of waterbottles for your family and will keep them all together, eg in a stroller basket, the ice will take longer to melt. But note that as soon as you add water to a block of ice, it starts to melt faster than if you had just frozen a whole bottle of ice.