Gather a lot of varied experiences. Read a lot of books, travel, talk to strangers, anything to open your mind to new ideas and ways of thinking.
Create a frame of topic. Addressing questions like: what is it you’re trying to do, and where should your focus be?
Ask a lot of questions. Who, what, where, when, why, how, repeat. What if this was like that? Where would this look out of place? Who would think this was brilliant? When will this not be true? You get the idea.
Use your imagination to answer the questions. How would a laptop work if it were a hat? Who from 1,000 years ago would have read this? What would this look like if it were 10,000x time bigger (or smaller)?
Write down or draw out your ideas. Do something to make the ideas more than just intangible thoughts. Writing them down is powerful, but so is vocally recording them or drawing them.
Filter the ideas down. Look (or listen) through your list of ideas and filter out the ones that are utterly useless or too far-fetched. Keep them around though, you never know when they will inspire something new.
Act! Now figure out the next step. Even if the idea seems radical, there’s always a way to move forward.
This is just one approach, but it works.