One way to give tattoo designs a cool 3-D look is to design the art as if it is tearing through the skin. A tiger or other animal tearing through the skin as if it is trying to escape the body, or leaving claw marks around it gives a design a 3-D appearance. Another example of a torn-skin look is a design depicting bones or mechanical parts with the borders of the design drawn to look as if a patch of skin has ripped away, revealing the metal or bones beneath.
Make tattoo designs of animals, insects and even people or other imagery seem to pop off the skin by adding a shadow to the design. A gray wash application of shadow to a spider or animal will make it look as if it is standing on the body, not just drawn flat in the skin. This principle of using gray wash, which is diluted black ink, to apply shadows to a design can be used with almost any shape or image to give it a 3-D look.
Tattoos that cover, enhance or interact with a person's bodily features will have a 3-D quality. For example, a man with male-pattern baldness might tattoo a tiny man with a lawnmower in his bald spot. The effect will look like the tiny man is mowing away the client's hair. Tattooing stitching, staples, safety pins or a similar design over a highly noticeable scar will give the impression that the tattooed image is holding the flesh together. While some of these designs aren't strictly 3-D in a visual sense, the interaction with the body gives them a sense of being outside or above the skin.
If you have a large body canvas available, such as your back, chest, thigh or other broad body area, try a perspective drawing, similar to the "down-the-street" view drawings we learn to make in school art classes. Adding a border around the design, such as a window frame, can give the image a sense of great depth. Tattoo admirers will get the impression that the design travels deep into the body.