Choose 300-count sheets instead of 600-count or higher. High thread count claims may be inflated or manufactured with thinner, lower-quality threads. Sheets made from Egyptian cotton or American-grown pima cotton are the best, regardless of thread count. They have the longest fibers, which mean they'll last the longest, and they'll save you about $100 per sheet set depending on brand and size.
Longer-lasting sheets, by definition, need to be replaced less frequently. Growing cotton requires energy, water, land and large quantities of agrochemicals. Processing, sewing and transport (most cotton items are made overseas) involve additional energy and resources. How is this good for you? If you're not buying super-expensive sheets, you're staying away from luxury linen departments—which can be major money suckers.