Fowler's is always a good guide, especially for general English. For writing for a publication, using one of the recognised style guides is even better.
For which and that, a general rule (not always correct but almost) is that "that" defines and "which" describes. As Sarah wrote, if you state that you have a watch, and add the information that it runs slow, you would use "which" with a comma. Another example is "This is the house that Jack built". "This is a house, which Jack built."