Visit tattoo studios in your area and ask to see the artists' portfolios. Because banner tattoos are quite common, most artists who have been tattooing for any length of time will have a few in their portfolios. This will give you an idea of what is possible.
Carry a notebook or sketchbook with you and jot down words that appeal to you. You may already know what you want on your banner tattoo, but if not, keeping a pen and paper handy at all times can help you remember your ideas.
Look at websites with pictures of banner tattoos and fonts. When you find one you particularly like, print it out and take it with you as reference material when you meet with your tattoo artist. The more reference material you bring to your artist, the better.
Ask a friend to draw a banner tattoo on your skin with markers. This will allow you to see how your design might look on your body.
Ask other people who have interesting banner tattoos if you can sketch or take a photo of their artwork. Again, this provides visual reference material for your tattoo artist.