Watch the proportions.
If you are wearing a knee or tea-length dress, consider a thinner sash. As you can see from the example above, a wide or elaborate sash would have looked out of place and distracted from the dress. This wide and dramatically bejeweled sash (below) would pair well with a longer dress so that the scale is more balanced.
Scale doesn't just apply to length, it can also apply to the overall design of the dress. This very delicate silver bow belt (below) wouldn't look right paired with anything overly-complicated. It would get lost on a frilly princess dress. Pairing it with a sleek sheath (long or short) ensures that both the dress and the sash will have equal time in the spotlight.
Juxtapose interesting textures.
You don't want your sash to overpower your wedding dress, but you do want it to get noticed. Make sure that you pick materials and designs that will stand out from the fabric of your wedding dress, while still coordinating with it. For example, this crystal and pearl starburst pattern (below) would go well with a lace trumpet gown that otherwise doesn't have much embellishment.
This simple lace gown (below) and ultra-soft organza floral sash is the perfect combo. The monochromatic effect works because of the different textures.
Likewise, you can pair a heavily-beaded floral sash with a simple, flowy gown (below).
When in doubt, go for sparkle.
In our regular, non-wedding planning lives, we would never go for something as crystal-encrusted and ultra sparkly as some of these sashes. But because they are sitting against the backdrop of an equally once-in-a-lifetime gown, this is an exception. Make sure to check out how sparkly sashes catch in the light, and make sure to try on a variety of styles and designs with your dress. That sash that looks too over-the-top in the case will probably seem just right when it's on!
Yes, you can wear these again.
Some sashes also can double as really cool headbands (you can even go for this option on your wedding day, if you don't want to wear a veil). Pick styles that are a little thinner and ones where the beading doesn't run the entire length of the sash.
Both this blush ribbon sash (above) and pearl beaded belt (below) would also look pretty fantastic as boho styling for a pair of boyfriend jeans and boxy blazer.