Guide to writing in plain language from The Royal Children's Hospital Melbourne [PDF]
Plain language style guidance from Office for National Statistics (ONS)
As you perhaps know by now, I love finding concise and practical style guidelines and here's another one for the list. This is the plain language section of the ONS style guide and it contains some sound advice and good links out to further reading.
Writing in plain English - South Tyneside Council
Style guide - A to Z - Guidance - GOV.UK
Of course, if you don't have a style guide for your organisation, you can always borrow some good advice from elsewhere. My freelance work is currently in the public sector on projects in local and national government, so the GOV.UK style guide is more or less my best pal at this point.
How to design a style guide that people actually use
Some useful insight here from Angela Moore writing on the excellent Scroll blog. If you've ever needed a style guide or you have one and no one uses it, this is a really good place to start.