Here's a hot tip straight from Tal Tomer, a seasoned and sharp Amazon entrepreneur.
When you open a new Display campaign, there's a new targeting option called "Similar to advertised products." Unlike category targeting, which had its share of issues (at least for my products), this is a more precise targeting option. It includes products that are identical in their product details to your product, meaning they use the same keywords.
In essence, this is a new automatic campaign that allows you to appear in premium placements within your most relevant competitors' product pages. With good ad copy and an attractive image, the chances of customers clicking through to your product page increase by tens of percentage points compared to regular ads.
I recommend starting with a relatively low bid to appear on the product pages of less dominant competitors and draw their traffic directly to your product.