Hi!
Here are some tips that we’ve learnt
First, be sincere.
Express appreciation truly and personally from your heart as much as you can. People can tell if you are not.
Second, focus on the people who will receive your letter. Do not talk too much about yourself.
Third, don’t have any hidden meaning in your letter. Do not try to make a sales pitch or get a favor.
Fourth, keep your letter brief.
Fifth, be specific.
For example, instead of just saying “thank you for your hard work”, add some more details that are results of one’s hard work.
Finally, try to avoid common expression. Instead be creative and personal.
Hope these can be helpful.