How often do you write them?
Do you go digital or old school hard copy? Long form or short?
What ever you do, let the person you are thanking know the specifics of your gratitude
What exactly did they do?
How did their action make you feel and;
What impact those actions had on you / the people around you
The more personalized the better
I have also found if you are going old school and long form, colored paper and envelop make a big impact - as does a hand written note. Note writing hack: if your handwriting isn't the best, I'd suggest you try an italic felt tip pen which should make an immediate difference to how it looks
Use the thank you note at year end to review someone's contributions; use it after a life event to recognize the support or help you have been given; use it "out of the blue" when your gratitude for that person or group rises in you. If you want to get better at this? Try a calendar reminder every 11 days on a repeat for a year - a five minute slot titled "Who are you going to recognize today?" and take 5 minutes to think of who to thank and write them a note...
Remember, no one ever complained about getting a heartfelt thank you note