Choose clothing in an opaque material. If it is high enough to cover your back tattoo, the material will hide it.
Use a flesh-colored bandage over a small tattoo. This will conceal your ink if you are wearing transparent clothing.
Ask someone to help you apply concealer if you cannot reach it yourself. The makeup should be about the same color as your skin. Apply powder on top of it after the concealer dries. This is a good way to cover a back tattoo when you are wearing transparent clothing. If the concealer/powder mix doesn't match your skin color, the clothing will make it less noticeable.
Make an appointment with a professional makeup artist. She will have concealers and special airbrush makeup that can hide tattoos extremely well. If you are attending a wedding, this is the ideal way to hide your back tattoo.