The center of the piece should be 60"-65" above the floor. If the ceilings are especially tall, you can go a little higher. Here is a good example of why not to use the "at eye height" rule as the couple is clearly different heights.
When you are mainly sitting in the room (such as a dining room), the art should be a little lower.
If the art is above a piece of furniture, such as a buffet, it should be 6"-12" above the piece of furniture so it all relates.
If the art is a tall vertical piece, hang it so the top third is at the 60"-65" height. This doesn't work if you are hanging more than one piece as in the picture above.
When hanging items in a group, use the lowest frame as the bottom of the grouping. If a row of 3 items vertically, use the middle one as the middle of the grouping.
If you are hanging art by yourself, cut a template from newspaper and stick it to the wall with painters tape so you can stand back and see how it relates to the other furnishings.
These are just a few of the "rules". I hope they were helpful. Do you have a specific dilemma or question with the rules? Put it in the comments. We can figure it out together.