While polyester blends don't wrinkle as much, they are not considered quality sheets. You want to get one hundred percent cotton sheets.
Quality sheets have a high thread count. Look for sheets that have a thread count of around 400. Anything higher is probably more hype than actual quality.
The best quality sheets are made from organic cotton. Many stores now offer this choice.
Often, but not always, quality sheets are offered by a designer who has a name to uphold. Designers also offer nice designs that are cheaply made, however, so look for good solid brands, such as Waterford.
Many mattresses today are very deep, so if yours is deep, make sure your fitted sheets have deep set corners so that they don't come off your bed. Conversely, if you sleep on a futon, look for fitted sheets that are not deep, or use a flat sheet.
If possible, feel how the fabric feels against your skin. Quality sheets will have a very soft, silky feel.