"Methodology" is used to mean a wide variety of things, many of which are independent of the programming language you use. Some low-level aspects can be language-dependent, such as code layout and commenting conventions. I'm aware of some shops that have tried to define their own in-house standards in such areas but I don't know of anything comprehensive.
Your phrase "trial and error development style" suggests you're focusing on the design-code-test phase (as distinct from requirements capture, estimating, etc). I think you'll find most experienced XSLT developers have a systematic approach to this, but I doubt that many could articulate it; it's likely to vary considerably with the nature of the problem. For example, it will be different if you're tasked with generating an HTML page that has already been designed by someone else, versus if you are prototyping the user interface design at the same time as you are doing XSLT coding.
I can't say I like the phrase trial and error. There are places for it ("Let's see what it looks like if I increase the font size"), but in general I think you should know enough about the input, the required output, and the tools that you are using, that when you write some code you have a good expectation that it will work first time.
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference
XSLT development methodology
