The obvious is never let a tree dry out completely otherwise it will surely die.
Watering correctly is an acquired skill, one that is learnt over the course of time. Your climate and weather being one of the many ruling factors as well as the positon of your tree ie sun or shade, is your tree in a bonsai pot/dish or on a slab, so many factors determine the amount of water that is to be given.
If possible, pick your pot/dish up when soil is dry, water then pick it up again, you will notice a vast difference in the weight of same, another guide to know whether your tree needs watering or not.
Make your Bonsai Garden more inviting with a water fountain from Fountain Crafters, read more