Being in a "fasted" state simply means that you have consumed no calories. It does not mean that you are not allowed to consume fluids. You may drink any zero calorie drink that pleases you while fasting. This includes water, tea, coffee, Crystal Light, and diet sodas.
Many people find that taking in their favorite calorie free drinks helps them pass the time, maintain energy levels and feel satiated while fasting. It is also very important to stay well hydrated during a fast since you are not getting the fluids that are typically contained in most foods while you abstain from eating.