Two things that are true about the habit of writing down what you’ve learned: 1) People agree that it’s the right thing to do. 2) Point #1 doesn’t matter, because they don’t like doing it anyway.
Writing takes time which could rather be spent on working. This is a myth, the second biggest misconception in the modern age (the first one is believing that cracking your knuckles actually causes athritis). Publishing takes time, writing doesn’t. Submitting a review on IMDB can take around 30 minutes, jotting down “nl2br() is a function for line break” only takes 5 seconds. I already measured both.
For tools of the trade, I highly recommend Notational Velocity (Mac) or ResophNotes (Windows). Creating notes is easy, almost with no barrier to entry. No titles needed, no manual saving, no organizing. Just write. To refer back on what you have written, just search (pragmatic programmer’s rule #2, search trumps navigation). The search bar and writing area is one, which will save you a lot of time. Search-as-you-type, which will save you a lot more time.
If you need more bells and whistles, try Evernote. This is what I’m using right now to record the solutions I’ve found while handling Anwers.
Don’t let your discoveries go to waste. The accumulated knowledge will prove to be very useful in the long run. Start writing today, for a better tomorrow.