Sometimes it's very easy to write, and the words just flow out. (Coffee helps for me, but you may have your own particular writing tools and environment that are particularly productive). It's important to capture these times as best you can.
Schedule work to post in the future. With good blogging software, you can capture a whole set of individual pieces of writing as separate posts, and then use the software's scheduling system to actually have them go to the public in the near future. If your Sunday night writing session is amazingly productive, instead of wasting all of those words on people the same night, you spread them out so that they are ready over the next few days.
Write things down that aren't ready to go. Writing ahead helps if you know that you need some editing help before the words are fully baked. Type things in as fast as the keys will help you out to get things down, and then save as a draft so that you can do back and fix up things that were only roughed in.
If you have enough of a buffer of half-done pieces queued up, you can always be ready to publish something at a moment's notice, hopeful some of it which actually has some thought to it. That's much harder if you are also trying to be timely or relevant, but hopefully some part of your world is predictable enough that you can work ahead and still be eventually just in time.