If you've got a retail site with a large product catalogue, you already know you need to make sure your customers can find what they need without too much guesswork. Your most motivated customers are the ones who use the search function on your site. In fact, they're twice as likely to convert because they know what they're looking for (source).
Tip: Not only will your customers find what they came for, you can use smart faceting to help them learn more about your product catalogue.
Tip: If you have a varied and large catalogue, facets can make the difference between a happy customer who comes back for more, and a would-be customer who gets frustrated and goes elsewhere.
Be relevant: Not all facets are created equal! Check your search logs to see what kinds of queries people enter. If you have a sales team or a physical store, talk to the people on those teams about the kinds of questions, concerns, and compliments they get from customers. Yes, you can and should use facets to educate your customers about the things they might not know (but might like to know) about your products, but facets should always be about helping them find what they want and get out fast. If someone has come with an intent to buy, don't hold them up with irrelevant sales pitches.
Make sure the number of facets is appropriate: Too few facets, even if they're relevant, may frustrate users. They're clicking the boxes so they can narrow their search results, so give them enough options to make this worthwhile. But you also don't want to add too many facets, which can overload your users, and, if you're selling a type of product that they don't buy often or aren't too familiar with, may make them anxious about making a wrong decision. Facets are about making them comfortable, not setting them on edge.
Weigh up the interaction cost: Faceted navigation isn't for everyone. If you only have a few pages of items, sorting or basic filtering on color, brand and category is probably enough. It requires more investment in your user interface and interaction design, creates a potentially more complicated user journey, and asks users to stop, think, and take further action after they've made their initial query. If you have a large product catalogue, this may be exactly what they'll expect to have to do, but if not, you might be slowing them down when they just want to get in and get out.
Tip: The most important thing to remember about facets is that they are about helping the user find what they want when they want it, not just saving them a few clicks.