Not to be flippant, but a back tattoo will only look good if you are a woman. That said, Sagittarius tattoos that are done with full spectrum of colors are very attractive on the back.
Well the best advice I can give you is to make sure you have a good artist. A good artist will work with you on the tattoo you want and come up with a unique design for you that will be the correct size and feel for the part of the body you are looking to use it on.
It all depends on your preference and what you like. What looks good is subjective. As long as you think it looks good. It depends on size, colors, design, etc
