Promote products at various price points. Even the little products (like Amazon ebooks) add up. If there is a truly useful product on the pricier side, it can still be worth the promotion even if only a few people buy it. If you’ve used a product of exceptional quality and it’s a good investment, or if it’s a product that’s unique, specialized or one-of-a-kind, go for it.
Always disclose your affiliate relationship. Most visitors will probably understand that graphic ads will lead to your getting paid, but if you write a review or use an in-text link as a recommendation, you want your readers to know that may lead to compensation as well. This ensures you retain transparency and trust with your readers, but also, it's required by the FTC's endorsement rules.