Words are simple and stark, and so long as they are inked clearly, they do well when inked on the tongue or lower lip. Choose a word that represents who you are and what you do, or simply a word that is what you would like to say to anyone with whom you are comfortable revealing your tongue or inner lip. While it is possible to get clear work in a cursive script, using clear block letters for high readability.
Stars are a good choice when want to tattoo your inner lip or tongue because they are small and at the same time they have an evocative shape. You can get stars that are simple line outlines, stars that are solid black or even stars that are colored. The moon is another good choice, especially if you combine it with a few stars. A crescent moon has a distinct shape that will look good on the tongue or the lip. If you love astronomy, a planet with a ring around it is another choice.
If you are a particularly loving person, or you want to remind yourself to always speak well of others when you can, tattoo a heart on your tongue or lip. The heart is a distinctive shape that is simple to recognize, and it is one that is easy to tattoo clearly. A single heart in the center of your lip or at the center of your tongue is a simple design, or you may choose something more distinctive and use several hearts.
Flowers are associated with femininity and beauty, and a simple flower does well on the lip or the tongue. Choose a simple, daisy-like flower outline or choose something with more petals, like a rose. You can add a strong blush of color to give the flower some visual variety or you can add a few green petals to add to the tattoo's intricacy.