Save The Lost Customers

As you already know, crystal clear navigation is important. However, clarity isn't enough to lead visitors to the right place.

People Think Differently
Categorizing is often used to navigate a large website. When you categorize your site, you need to clearly indicate what each category is about so that visitors can pick the right one. Most websites are doing a good job on this. The problem is you and your visitors don't think a particular product can be found under the same category.

An Example
Let's say you are looking for a memory card at amazon.com. Which category would you pick from the drop down menu for searching -- computers, electronics, camera & photo, cellphone & services, or what else? People will pick different categories. That's not because people are confused (Amazon.com clearly indicated each category); it's because people all think differently. People will pick a different category based on what the memory card is for -- digital camera, mp3 player, cellphone, etc.

See The Difference
If visitors cannot find a product under the category they think the product should be at, they will think you don't carry the product. To avoid this, you need to find all categories a product could fit into, rather than find the best matching category for a product. Then, list the product in all of those categories. You could cross-refer other categories, i.e. adding see-also links. But I prefer listing the same products in multiple categories. At amazon.com, you can find memory cards in multiple categories, so visitors can find the products no matter what category they choose, and amazon.com is not losing any sales.