Color - Make sure there is good contrast between the pattern of the QR code and the background. If the color is too light with little contrast, it will make it difficult to scan.
Formatting - When incorporating a QR code into your design, treat it as you would a regular image. Make sure when you're resizing the QR code, scale it proportionately. Don't stretch or distort the code.
Size - While some customers have successfully printed extremely small QR codes on their products, we recommend keeping them at least 1" in size with 1/8" white spacer around all sides to ensure maximum readability.
The smaller the QR code, the closer the user's camera must be to the printed item. Minimum focal distance of some camera phones can be 3-4". The more complex the code, the larger it should be. Shorten the URLs – use something like bit.ly or goo.gl. This reduces the amount of data the code needs to hold and makes it much easier to scan. An additional benefit to URL shorteners is that they generally have some tracking capabilities.
Medium - Think of the medium and how it's going to be viewed. It can be smaller on a business card, but needs to be much bigger on a vehicle magnet. Rule of thumb: 10:1, a QR code that is 1 inch (25.4 mm) could be scanned about 10 inches (254 mm) away.
TEST, TEST, TEST - Before submitting your file, print out a hard copy and test it in different lighting scenarios. Make sure the item you're linking to works and displays properly on as many phones as possible.