Use filtered water, possibly bottled. Though this is the cheaper and healthier way, there are long-term scale preventing solutions as well.
In these cases, it is important to repeat the methods weekly or at least monthly. One can use a mixture of vinegar and water or with scale solvent. If using scale solvent, pay high attention to the instructions.
When purchasing a household appliance, check the product how easy it is to clean, including the removal and/or prevention of scale.