How big are your ears? There is no standard sizing for earplugs, and most are one size fits all, although some manufacturers sell them in small, medium and large. Try a variety of earplugs until you begin to get a general idea of your ear and ear canal size. As a heads up for women, most earplugs are sized for men.
What is your intended use? Do you need to block noise for eight hours while you sleep? If so, comfort should be your top priority. Otherwise, look for earplugs that are specifically designed for the activity you are going to be pursuing. EarPlugStore.com has a great breakdown of which earplugs are best for which situations.
Do you know how to properly insert foam earplugs? It's important to know how to properly insert earplugs to ensure peak performance. Most manufacturers have either written instructions or a video tutorial on their website.
Buy a variety pack. Before you decide to invest in one earplug or another, get an assortment pack and try them all. Some manufacturers sell assortment packs of their earplugs.
Take them for a test drive before buying in bulk. The best way to decide if earplugs work for you is to fully try them out and not just put them in your ear and declare them comfortable. Try to mimic the activity you'll be using them for, wear them for extended periods of time and most importantly move your jaw around by chewing, yawning and talking. As you move your jaw, it can change the shape and size of your ear canal, potentially turning comfortable earplugs into painful ones.