Law school teaches young would-be-lawyers a lot of things but there are a lot of basics that they don't teach law students at all. One of them is how to write an effective letter to a client. Historically the most common grievances come from lack of clear attorney-client communication skills.
After awhile, letter-writing gets to be automatic but until you get there (and as a reminder even if you are already there), here's some tips on what a client letter should cover, in the order they should appear in the letter.
1. The Why. Tell the client why you are writing (to update them on status of their case or an issue, to ask a question, to provide an answer to a question they asked, to send them documents to review, etc).
2. Just do it. Provide the update, ask the question, give the answer, explain the documents, tell them what you want them to do, etc. Do it.
3. Get a response. If your letter requires a response from the client, tell them how (via email, phone, etc) and when you need their response (a firm deadline). If your letter does not require a response, then say that (so the client doesn't wonder if they are supposed to respond). If a conference with the client is needed, then ask the client to make an in-office or on-phone (say which way you want it to be) appointment to discuss the issue and their resulting response.
4. Explain it. Tell them why or how the immediate inquiry fits into their case at that moment, so they understand why it matters and why it is important to respond.
5. Predict the future. Tell them what will happen next in their case, so they realize that you are working on their case and that you need their response in order to proceed to the next step of the case.
6. Say something good. Use the opportunity to say something positive or provide reinforcement to the client. Your letter may cause concern so your client will appreciate hearing some "good news" in their case, even if it's only your opinion or reassurance that things are proceeding normally as far as you are concerned.
7. Express thanks. Thank the client for their continued patience and assistance and say that if they have any question to let you know and how to contact you (phone, email, etc).
More tips on letter writing:
Generally, your message should be sent in the manner that you expect a response to occur (if you want them to email you, then you use email to send the message, etc). However, always remember that some things require the personal touch of a phone call.
The letter should generally be less than one page or two at the most. Everyone knows that lawyers sometimes like to talk at length. Lawyers often fail to realize that non-lawyers don't like to read (or listen) at length.
If appropriate, a copy of the letter should be put in your follow up tickler system as a reminder for a specific future deadline date.
First-timers need to remember the above. Later, after years of letter writing, you may need to remind yourself again. Good client communications are critical for long-term professional success.