One of my favorite ways to stay sharp is to keep a notebook of various topics. Once you've developed the habit of keeping a notebook, you'll have an easier time coming up with topics to write about, and you'll even have a ready-made list of references for punchlines, which can really help speed up joke writing.
A notebook can be filled with all kinds of topics:
Hit up your favorite newspaper or news website for the latest updates to Planet Earth. Don't just stick to top stories - look in the Life section or even the comics and write down the broad topics you find there. For example, Easter's around the corner and maybe you've seen some articles about Easter egg hunts or Easter baskets. What else relates to Easter? Use assosciation to come up with potential ideas.
Every once in a while, you'll be going about your daily business, only to stumble upon a peculiar sight. For me, it was the time I was standing in the mall and noticed a toddler in a carriage handling a pack of cigarettes while his mom was looking at jewelry. Before telling your friends about it, write it down! You'll never know when you find a use for it down the line.
The Daily Routine
Mind you, not every day can be graced with a strange encounter, but normal days can feed the notebook just as well. Think about everything you did yesterday from morning to night and use those events to come up with topics. For example, if you had cereal for breakfast yesterday, some topics stemming from that could be "I love how easy cereal is to make" or "cereals I would invent".
Opinions and Beliefs
We're all hopelessly biased in one way or another. Fortunately, that makes perfect notebook food. Weigh your current thoughts and beliefs from an objective standpoint. Which opinions make the most interesting topics? What might other people think about your beliefs? How would they react? An introspective adventure might serve up topics you would have never thought about at face value...you just need to be honest and open-minded.
These are but a few ways you can fill the pages of your notebook with topical goodness. Do you know any other ways to come up with interesting things to write about? If you do, go ahead and drop me a line in the comments.