Add a water bed conditioner to the water you will be using in your mattress before you fill the bed. The conditioner can be purchased at a mattress retailer. Using it will help keep the water in the mattress from stagnating and smelling bad. Empty out and refill the water once a year. Purchase a drain-and-fill-kit to empty the mattress and replace the water. Add conditioner to the water before you refill the mattress.
Use a mattress pad; place it beneath the sheets you will use for your water bed. Mattress pads help keep oils from your skin off of the vinyl of the mattress and keep it from being damaged by them.
Clean your mattress with water bed vinyl cleaner once a month. Vinyl cleaners will help condition and maintain the quality of your mattress.