What size are your ears? There is no standard sizing for earplugs, and most are one-size-fits-all. However, some brands come in small, medium and large -- a good starting point if you know that your ears are on the large or small side. Try a variety of earplugs until you have a good idea of your ear and ear canal size. As a heads-up for women, most earplugs are sized for men, so you're more likely to get a good fit from a smaller size.
What is your intended use? If your goal is to block noise while you sleep, you need earplugs that are powerful enough to muffle ambient noises, from traffic to your partner's snoring, and comfortable enough to wear for eight hours at a time. For swimming, your main priority is whether the earplugs do a good job keeping out water. If you want earplugs to wear to concerts or band practice, you need a set that can soften sound without distorting it -- like turning down the volume knob on your ears. For riding a motorcycle, look for earplugs that fit comfortably and securely under a helmet and block out loud wind noise, while letting through enough traffic noise to keep you safe.
Do you understand how to insert earplugs properly? Most manufacturers provide either written instructions or a video tutorial on their website. Taking the time to practice the recommended insertion method can make a world of difference in how well your earplugs work.
Do a thorough test. Before buying any brand of earplugs in bulk, you should take them for a thorough test drive. Simply putting them into your ears and seeing whether they feel comfortable isn't enough. The best way to decide if earplugs really work for you is to try to mimic the activity you'll be using them for, whether that's sleeping, swimming, using power tools, or something else. Keep the earplugs in place for an extended period of time and see whether they stay comfortable. Also, make sure to move your jaw around by chewing, yawning and talking. Moving your jaw can alter the shape and size of your ear canal, potentially turning comfortable earplugs into painful ones.