After the iPad was announced -- but before it actually appeared in stores -- a friend of mine here in Denver created some mock iPads made by printing up a full-size image of an iPad and then gluing it onto Foamcore. The result was convincing enough that I walked into an Apple Store with the mockup and got more attention than I wanted from employees and customers alike. Well, now you can see how an Apple Watch in any size and with most bands will look on your wrist, thanks to ExactFitnessApps.
Some brilliant person at that company (developer of EXACTivity Pro, Exact Fitness Timer Pro, FitCalc Pro, and Fitness Spades) worked up a PDF file that, when printed out at 100% scale, gives you exact-size mockups of the 38mm and 42mm Watch with the Sport Band, Modern Buckle, Milanese Loop, Leather Loop, Link Bracelet, and Classic Buckle bands.
Where this is really helpful is with the Leather Loop, Modern Buckle, and Sport Band, which all have bands in different sizes (the other bands are one size fits all). Of course, the paper replicas are only as thick as a sheet of paper and they're black and white, but borrow some crayons from the kids and have a good ol' time making your mockup look more realistic. Be sure you check your print settings to make sure you're printing at 100% scale. My first batch seemed small... and had been scaled to 92%.