I've been writing funny copy for years and even have a library of funny radio spots to my credit. However, it wasn't until I won a stand-up comedy class that I really thought about the mechanics of humor. I learned to write jokes and that new skill improved the writing I do for clients.
You may never find yourself alone on a stage telling jokes to slightly inebriated stangers (something I find oddly relaxing), but do invest some time to learn how funny works.
A great place to start is "The Secret of Writing Funny" by Annie Binns. Try her six techniques and you will be funnier—at least on paper.
If you know of other resources that have helped you write funny, please share them in the comments.