There is a range of options depending on the product's value and its risk of counterfeiting. Casual counterfeiters (a.k.a. "digitfeiters") can be deterred more easily. An unusual substrate that is hard to obtain can be used, as can an overprint coating. If printed digitally, a clear toner may be added on some presses that fluoresces under UV light. A fake watermark can also be printed on the reverse side of the substrate that is not obvious to a weekend fraudster. Higher counterfeiting risk calls for more sophisticated and covert devices. Using several devices at once is necessary. You can use fine-line security patterns (a.k.a. guilloches), digital or paper watermarks, void pantographs (to stop copying), and various microdot and difficult-to-reproduce geometric patterns. You can also purchase paper with embedded fibers and other security features, as well as specialty ink that color shifts or iridizes. Many of these sophisticated techniques are licensed, requiring you to obtain the technology and know-how from companies specializing in security features. Also, using security devices expected to thwart professional criminals brings significant liability and a tough learning curve. In those instances, consider hiring a security printing consultant or leaving the job for a bona fide security printer.