There are many retro-reflective products out there, but these items from FlashBack are simple, inexpensive, and easy to apply. And, hey, those flag stickers are really cool!
The products come in a variety of sizes and shapes; you can get plain 1" wide lengths of tape, which you can cut to different sizes and shapes. Or you can get 1/4" pin striping tape, which works great on helmets or to outline saddlebags.
The FlashBack products are high quality, and made from 3M reflective stock. They contain over 45,000 micro-reflectors in each square inch (6 cm2) of material.