Reference to the posting (usually on the posting order).
Explain a bit about yourself and your military background.
Include any hobbies or interests that may be of benefit to the receiving unit.
Include details of your family and any employment your spouse hopes to undertake.
Always finish by saying how much you are looking forward to serving at that unit (even if you are dreading it), remember you dont get a second chance to make a first impression!
Be brief and dont waffle on, 3 paragraphs is usually enough to get your point across.
Anyone worth their salt will reply, in writing, to you, it really annoys me when people cant be arrsed to put the effort back into you.
Sorry to be a bore..... Are you joining the unit as an officer, a WO, a SNCO or a JNCO? I'm assuming the latter as the other groups should (flaming well ought to) know the form.
If the latter,
I have the honour to report that, by authority of <Insert the posting order date and freference) I am posted to your unit in the appointment of <Insert the deathless post you will be gracing the unit with>
I have previously served with <insert carefully selected plum posts and units>
I have been in contact with <Insert your immediate boss> and with the Garrison housing authorities (This reassures your Guvnor that you are not an admin 'mare).
As an Army/Combined services/national/International ski/rugby/ basketball player I am delighted to have been posted to your unit where I hope to (select as you will)............
I am greatly looking forward to my posting to XXX and to serving under your command.