Get to know your shells - The more you learn about shells the better you will do. At first you can't help but pick up every pretty shell you find. The more experienced shellers don't pick up a lot of shells. They are into quality not quanity. Get yourself a good shell identification book. I like anything by Conchologist R. Tucker Abbott. For smart phone users check this shell identification app out.
Online Sites -
The Bailey-Matthews Shell Museum
Don't take living shells - Resist the temptation. In in earlier days that was how people shelled. If we want to continue to find beautiful shells & sea life we must leave the live ones behind to make more shells. It is against the law and carries a $500 fine if caught. Take a picture with your camera and enjoy that instead.