Carry a sketchbook or journal with you and fill it with images and ideas for your tattoo. When you see a person with ink you like, ask if you can make a quick sketch in your book. Write down recurring themes you notice in your life and make drawings to go with those themes. For example, if you really like cats, sketch real cats, cartoon cats and the cat tattoos of others.
Visit tattoo websites to get ideas. Take note of what makes a good tattoo good and a bad tattoo bad. Note line thickness and sharpness and the way different colors look on skin tones similar to yours.
Draw on yourself with magic markers or have a friend draw on you. Pay attention to how the drawings move with your skin. Use permanent markers so you can keep the designs on your skin for a few days, observing them in a variety of situations.
Visit tattoo shops in your area and look at artists' portfolios. All reputable shops will have books of the artists' work available for customers to peruse. Talk to the artists, if possible, to get a sense of what kind of work they enjoy doing and how your idea will translate into a tattoo.
Take as much visual reference material with you to your tattoo appointment as possible, and trust your artist to redraw your design to make the best tattoo. Listen to your artist's suggestions and be willing to accept her advice. A good tattoo is a collaboration between the wearer and the person he entrusts to permanently ink the design onto his body.