i have the following tips for any thank you cards...
1. make them personal. try to say something different for each of your LOR writers. this is very important, in my opinion. and, also protects you if you ever get cards mixed up (this happened to me, and i had to go make the exchange and i was ever so glad the cards were unique to each professor 😉)
i thanked my bio-lab professor for his guidance through all my labs and his patience with me, my research advisor for always taking the time to support and encourage and guide me, etc etc. The theme is similar, but change up the adjectives and its an added bonus if you can add something specific to your relationship with them. this works even better if you have a good relationship with them: i did, and it wasn't easy for me to think of things that would make a connection.
2. keep it simple. don't be too wordy or flowy, and be careful with the words you use. you don't want to sound fake or insincere. use words you would usually use.
3. thank them for the time they have taken out from their busy schedules to write you an LOR. thank them for their confidence and belief in you. tell them how much it means to have learned from them, and how grateful you are for their support/guidance/mentorship/advice/encouragement.
4. If the card is to someone very important and it has to be super-formal, write out something rough and tweak it, and then read it to someone and see how it comes across to them. this may seem a bit much but in my experience thank you cards are very important, especially to LOR writers.
5. there are a lot of examples online for thank you cards in case you're really stuck. take a look at a few and then it'll be easy to write your own.
i also believe these cards should be handed in person whenever possible. dropping it off in the mailbox just doesn't have the same impact.