The AIS Iris Encyclopedia (also known as the Iris Wiki because you can edit it) has a Newsletter Resources page for all newsletter editors. This has pointers to a number of American Iris Society resources and others you may find useful.
Thanks to Kathleen Sonntag, the Irises Bulletin editor, one of these resources is the style guide used for Irises. It is available as either a PDF or Word (docx) document by following the links below. Eventually we hope to have it as a page on the Iris Wiki so everyone can update it.