It's the little things
The Shane Company has a great example of a slick usability trick in their print-out ring sizer, for you to see which size ring you need, they have pictures of coins and a ruler on the printout so you can ensure that the printout wasn't scaled to an inaccurate size:
Very simple, but very nice.
The printout has many other simple but elegant tricks on it to ensure that the user doesn't get anything wrong, such as:
Guides so that you cut out the ring sizer at the exact tip:
Printing the numbers in the center so that they're not mercy to your scissor skills:
And if you have a ring that fits perfectly, you can just compare that ring to the circles on the page to see what the perfect size is... it's always nice to provide two different ways to perform a task if you have two different entrypoints through which you might need to get to the answer: