First, they think of a broad topic they would like to use and write it down. Then, I ask them to come up with as many questions about the topic as they can think of. When that's done, they do some preliminary research or freewriting to come up with an answer to that question. That answer then becomes the controlling idea, and the thesis comes from that.
For example, if my topic is texting and driving, one of my questions is "Is it really that dangerous?" I find out from research that reaction times are similar to drinking and driving. So my thesis statement is my topic, plus my controlling idea and opinion: Texting and driving is as dangerous as drinking and driving, so the penalties should be the same.
I put this idea with a worksheet, along with everything else I use for helping students write good thesis statements (handouts and worksheets, plus a power point) in this new packet. It's in my teacherspayteachers store for only $3.00. Happy writing!