A QR code that looks great on screen can be unusable once it's shrunk down and printed on a flyer. A little planning at the design stage avoids a lot of wasted print runs.
Size it for the distance
A code scanned from a foot away can be small. A billboard scanned from across a parking lot needs to be considerably larger. As a rule of thumb, the code's width should be roughly one-tenth of the expected scanning distance.
Leave a quiet zone
QR codes need clear white space around their edges to scan properly. Don't crowd the code with text or images right up to its border.
Choose the right finish
Glossy paper can create glare that confuses a camera. Matte finishes tend to scan more reliably under most lighting.
Always include a call to action
A code on its own doesn't explain itself. A short line like "Scan to view menu" or "Scan for 10% off" tells people exactly why they should bother.