There is not a direct analog to "copyleft" (informal but widely recognized term) in the world of patents.
As mentioned by the two previous answers, if your sole concern is ensuring that no one else can patent your invention, then public disclosure of the invention is probably your best course of action. However, you would want to make that disclosure as widespread as you are capable of doing so that patent examiners are readily able to find your invention to show it as prior art against anyone that attempts to file for a patent on your invention. Simply posting it to your blog is probably not going to cut it.
You ideally would want to get it published in some sort of widespread journal with a large distribution, and publicize your invention like crazy so that there are multiple venues through which an examiner would run into your invention. Of course, that course of action is not always feasible.