Some of the best information I have heard about how to name variables, methods, etc. comes from the Stanford MOOC on iTunes: Developing iOS 10 Apps with Swift.
In the first couple lectures, the professor talks through how to build a calculator app (and does so in front of the class in real time) and does a fantastic job of demonstrating why he is naming things the way he is. The essence, as he explains it, is this: a statement in code should read like an English sentence.
One of the early requirements for the first assignment is to read through the Swift API Guidelines. Admittedly, this is designed for the Swift language, but there are lessons to take away from this for any language. The naming can be verbose at times, but such verbosity might improve code comprehension as students will have to know what a variable or method does in order to name it properly.
Even just the following two would be a great start for most students:
"Clarity is more important than brevity."
"Write a documentation comment for every declaration. Insights gained by writing documentation can have a profound impact on your design, so don’t put it off."