Practice and processing of that practice....is the simple answer.
We practice it a lot.
We take a topic and start thinking about questions that are interesting...and then they start brainstorming a big list. Topics trend up and down...and the longer the brainstorming goes the better the questions get. Students hear a good idea and use that good idea in building their question.
We process a lot.
Once we have a pretty long list and everyone is just repeating, we start processing. I ask them what are questions that are easily Googled and which ones are ones that will require more thought. It's a simple filter...but a good one. It's actually a skill that is sort of invisible but very useful if you stop and think about it.
Ususally the processing ends up with min-conversations about what they think or where they'll go to find out more. I believe it's a great way to incorporate vocabulary development....just slipping the words that I know as their teacher that are relevant to the topics. Sometimes we leave it there.
Sometimes we save the list because we use it throughout the set of lessons. Answering them. Keeping them on the unanswered list. Using them for inquiry projects.
Practice and process. The simple keys to getting better at asking quality questions.