- Try to avoid over formal format, if you writing for someone then you should write as how both of you talk to each other in real life. You'd want this letter to feel as genuine as possible
- Don't use the overly bombastic word, you would want the message as simple as possible so that the recipient could understand the message you trying to convey in one glance
- Think up some quote or search the internet for a quote to encourage the recipient to keep up his/her hard work, don't forget to give a compliment for whatever he/she has accomplished.
Sorry if the information is wrong or doesn't suit you, this is based on my experiences.