Write it to the OC Wksp/LAD not the ASM - I don't know where this habit of JNCO writing to the ASM started but it is the OC's unit not the ASM's (if you're a senior write to both the CO and the OC).
Of course the letters will be passed to RSM/ASM respectively for action but the point is they wont mind and the OC will have you on his radar (assuming of course you're an asset not a liability!!)
Start with a fully formal:
I have the honour to report that in accordance with RMO posting order No 123 dated 010203 I am posted to xyz.
Tell him if you're a singly or accompanied and that you've sorted you admin out (not in these words)
Tell him a bit about yourself, sports, interests etc.
Ask him what's coming up in near future etc.
Tell him where you've served, what you want to achieve whilst in the new unit.
Sign off again with a nice formal,
I have the honour to be,
your obedient servant
Now I'm sure somebody will come on and tell you there's no need for this level of formality, it's old fashioned etc. but what have you got to loose?
Better to score points than make yourself look an arse and start off on the wrong foot.
Write (at the very least) the salutation and signature in ink, on decent white paper and matching envelope and for pity's sake get the Unit name right. Two pages max!
The only thing I would add is that if you are at all unsure about your letter ask some one such as your platoon commander to check it before you send it. As Mr Brown found out a badly written letter can have negative consequences.
If you are on DII you could send an e-mail to your new OC and copy it to the ASM. Make sure it is correctly formatted as per JSP101 etc. You could even attach your CV to it if you have one.
Cheers for all the help gents, think I'll go with a mix of oldfart and smudge and send it in a text just so they know what to expect. "Alright m8, pstd 2 ur gaff cya l8trz jay" should cover it I think . The SSgt I spoke to said just the PC and ASM as the posting is an OPR coy and the unit is spread all over the place with HQ in Wrexham
Indeed, as did I whilst a junior rank. I only wrote to my new unit's CO, RSM and the LAD OC when I was a SNCO. Not sure when the habit of Junior Ranks writing in advance started to gain popularity but it was never expected nor frowned upon that I didn't when I were a lad