I know we pretty much all thank our parents when they buy us dinner when we're back home visiting or when they spontaneously pay for new clothes or groceries, but to be honest, that isn't enough. Doing only that is just like going through the motions. Obviously, you should thank your parents for when they do something on the spot because that is the norm and expected, but I'm not writing to tell you to do that more often.
Parents do so much for their children throughout their whole life. When you thank your parent as a younger child it isn't the same as when you thank them when you're older. As a child, you aren't aware of how much your parents actually do for you or how much they affect your life. Once you enter adult years, something changes. Have you ever done, seen, or read something and in your mind you thought, "Wow, my dad/mom was so right?" I know you have, don't lie to yourself. Your parents have prepared you for the life that is ahead of you. Without them, you wouldn't be where you are today. It's alright if you just now realized all they have done, but if you have known for a while, it's time to say thank you.
All those times you rolled your eyes because your parents always said the same things over and over again. All those times your parents didn't let you do something when you were younger and you got so mad at them. All those times they told you, "one day you'll understand why." Now you do, so go, go thank them. Not for buying you food, or letting you stay in your room still when you come home. Those "thank yous" are everyday ones. Go thank your parents for shaping you into the person you have become and are still becoming. Go thank your parents for not letting you go out on a date with a boy when you were only 11 years old. Go thank them for "ruining your life" during your teenage years. Go thank them for always wanting the best for you.
You may not have been aware of their doings being for the better when you were younger, but now that you are an adult and see the reasoning behind them, thank them. They deserve it. They've been waiting for you to say it, so do it now! And don't forget to always remind them how much you love them.