Like ThermionicScott said, look up the guidelines for reflective gear on RUSA: Randonneurs USA. For those who don't know, RUSA sanctions long-distance, minimally-supported rides that often go all night, so lighting and reflective gear is important.

I've had many, many opportunities to see all different types of reflective gear in action on overnight rides, as a rider and also as a volunteer (where I get to see the riders from a car's vantage point). When you're riding in the dark the single most important factor is reflective surface area. Color doesn't matter much (except that light colors are a bit more visible than dark ones). In other words, a sash that is 2" wide in neon green and has a 1/2" reflective stripe is basically no better than just a 1/2" reflective sash. Sam Browne belts can work well, but the part of the sash that attaches to the waist band should be on your left and it should go over your right shoulder so that the most visible area is on the left side of your body. Riders wearing the sash over their left shoulder don't show nearly as much reflective material to traffic coming up on the left. Make sure the waist belt is also reflective, and that it is not flipped upside down or caught up under layers of clothing.

Some vests and jerseys have small, graceful amounts of reflective piping, reflective logos, etc. The label says things like "reflective accents for safety". But if it's just a bit of piping around the shoulders and a narrow stripe on the pocket, it's not good enough. It doesn't matter how miraculously effective the specific material used is, and it doesn't matter if the whole vest is hi-vis green; it's just not enough reflective surface area. Period. Reflective surface needs to be measurable in tens of square inches, not little touches here and there. That's not as much to wear as it sounds like.

The Amphipod things generally are reasonably visible, except for riders in an exceptionally aggressive position, which most aren't in the city or in the middle of the night. My other complaint with them is that they don't show much from the side, either. But they're generally better than a lot of things, and better than a Sam Browne belt that isn't worn optimally.

Those cheap highway safety vests aren't designed for cycling, but they do have substantial areas of reflective material and they're always very visible. They are also generally big enough to fit over jackets, camelbacks, etc. They're big and sometimes hot, but keep in mind that the parts of the vest that are not the reflective stripes are doing practically nothing for you, comparatively. It's possible to wrap just as much reflective material around your body without the excess vest.

BTW, "Illuminite" is a surface created by printing reflective surface onto fabric in lots of little blobs, instead of in solid areas. That makes it good for covering large surfaces, because the little blobs add up to substantial surface area while still being able to breathe, but it's not as bright as a solid reflective surface would be. There are other similar products out there, but I'm skeptical of anything like that unless it's used on at least 50% of the outside surface of a garment. It also tends to not last as well because the little blobs can get rubbed off. The same is true of any time a reflective surface is applied to a stretchy material - when the fabric stretches, it gets broken into little pieces which can rub off more easily.

One more thing you can do, if none of these solutions appeal, is buy reflective fabric and stick it onto whatever you need to with safety pins. You can either buy it in the form of sew-on tape of various widths, or you can buy it as fabric by the yard. Some good sources are Identi-Tape - All Kinds of Colored Tape , Sunbrella : Grommets : Clear Vinyl : Marine Vinyl : Outdoor Fabrics : Seattlefabrics.com , or Outdoor Wilderness Fabrics - Supplying all your outdoor fabric needs . I've bought stuff from all these places many times.

Again, while some reflective surfaces are better than others, the overriding factor is still total surface area. (And it never hurts to have more of it on your bike, your bags, your ankles, etc.