From LabVIEW Wiki
Originally there existed a document entitled "LabVIEW with Style" -- A Guide to Better LabVIEW Applications for Experienced LabVIEW Users", authored by Gary W. Johnson and Meg F. Kay. Consider this document to be nominally obsolete. It is on the CD-ROM with the book "LV Graphical Programming" but was lost from the Info-LabVIEW ftp site with a great directory shuffle there some years ago (it was in a now-absent DOCS directory). The document does not appear to be on the NI ftp site either.
There is a much better reference available now. If you buy the Professional version of LV6.0i, you get a printed manual called Development Guidelines. Even if you just get the non-pro version, install all the PDF manuals and you will find the same document as LabVIEW Development Guidelines. It's a wonderful little book, written by the VI Development team at NI with input from many... including that ancient style guide that Meg and I worked up. Highly recommended.
Also on the NI website a search for STYLE GUIDE will locate the "Rules To Wire By" series by Rande Johnson. It's a great series, and may well meet your needs. Lots of wisdom there.
There are more works in progress on this subject, including one or more books (NOT by me!). And NI will keep revising the Development Guidelines book.
One book that came out recently is "The LabVIEW Style Book" by Peter Blume.