Whether you get on like a house on fire or they drive you bananas, you musn't forget they've given you the best present ever - the gift of life itself! So don't forget to thank them from time to time for their sacrifices and, if you have a great relationship with them, for their unconditional love.
Use any way you can to express your gratitude towards your parents: a kiss, a hug, even silent moments of affection or a note on the fridge! They'll really appreciate it.
And if you live far away or you don't see your parents much, you can always thank them in your heart and you'll be at peace with yourself.