Here at BluegrassDaddy, I endorse the "Don't Fret" sticker. This finger position indicator was created by Phil Coonce, who is also a BluegrassDaddy.com subscriber. I highly recommend this sticker for beginners. "Don't Fret" stickers come in a variety of sizes, so be sure to select the correct size for your fiddle. Click here to purchase your own "Don't Fret" sticker.
From Phil Coonce:
Don't Fret, the patented finger position indicator will help you solve these problems. Inexpensive, mathematically designed for accuracy, and made of clear, self-adhering vinyl, it applies easily to the fingerboard. Colored lines arranged in a scale pattern show the students exactly where to place their fingers. It gives you a resource for teaching finger positions, scales, keys, and intervals. Whatever your method, Don't Fret helps you communicate more effectively. So work hard, but Don't Fret!
Here is a video with detailed instructions on how to apply the "Don't Fret" sticker: