When preparing a bath, it is important to swirl the water during the whole of the preparation of the bath, to get an even heat around the bath, but not to enter the bath until it is full. The temperature of the bath should remain constant throughout the bath, so every 5 minutes or so add some hot water. Don't wait too long, and then add a lot of hot water at one go. Your body should be immersed in water of even temperature throughout.

Feel free to add bath oils to the bath. However, be sure to mix them correctly. Don't mix oils for soothing muscles(such as Marjoram), with erotic oils (such as Ylang ylang), for example. Only use oils which are compatible with one another. Don't use oils in the bath if you intend to take a bubble bath, except for Eucalyptus, and, possibly, Orange, on a sunny day. If your bath receives direct sunrays, avoid Mint in any case.

Don't bring any animals into your bath with you, especially not if you wish to wash them. Except of course a fish, and then it should only be a fish that has lived with another fish inside a fishtank for at least 24 days. The fish should not be larger than the palm of your hand.

Once in a while, you can put your entire head underwater, tilt it back, and make a "whooshing" sound with your nose. When your ears are underwater, you can hardly hear anything outside. When taking a bath with another person, make sure they are not a member of your family, or you could get arrested. Also, try to pick someone you will enjoy taking a bath with. Don't just take a bath with anybody.

Bubbles in a bubble bath should be between 0.5" and 1.2" wide. Make sure all your bubbles are that size.

Most importantly, and surprising that prole and TheLady forgot: you should be naked while taking a bath.