Tip 1: Adequate pocket depth- The fitted sheet should be deep enough to fit your mattress. This is to keep it from sliding or popping off. Now, I have had this to happen to me, and trust me, it's frustrating.
Tip 2: Breathable fabric that works for most seasons- Sheets that are 100% cotton "breathe" more and are less likely to make you sweat than polyester blends, which is especially important during warmer weather.
Tip 3: Reasonable care instructions. With the exception of very high-end silk, mostly all sheets are machine washable and dryable. Follow the manufacturers care instructions for cycle speed, temperature and stain removal to maximize their lifespan. Manufacturers give instructions for a reason!
Tip 4: Usually the higher the thread count, the softer the sheets will feel. This may not be true in all cases because fiber quality, and yarn size, play a role in a sheets comfort and durability.
So if you are deciding on which sheets to buy, check out my tips above. Happy Shopping!