“Which” is slightly more high-falutin’ than plain old “that,” so people, especially in business, tend to use “which” when they should use “that.” Let’s strive for clear communication, here, folks, not snootiness.
Which—are you making an aside, a remark that supplies extra (not necessary) information? Then use a comma and “which.” For example: “The steamboat, which was chugging across the harbor, capsized.” Sorry for the grim example, but it’s made up, so no one was hurt. All that “which” does is add some information about the steamboat. The sentence would have the same meaning without it. “The steamboat capsized.”
That—Are you giving extra information that’s necessary? Then you need “that.” Suppose you had several steamboats, and you need to identify the one that capsized. “The steamboat that was chugging across the harbor capsized.” No commas, either.
Bonus info: You don’t have to use commas to set off the aside. You can use parentheses or dashes. For example: “Correct use of asides (which I hope you learn) makes your writing more lucid.”
Extra bonus: If you use MS Word, and have the grammar checker turned on, it just about always gives you the correct recommendation on this one.
Maybe you can come up with better examples of this. Put them in the comments, and if you haven’t already, download my five basics over there on the right.