JUST START SKETCHING
Our brains need to be triggered to work on a problem and sketching is a perfect trigger for designers. Just start sketching the first thing that enters your mind relative to the problem you are working on. Don't worry if you make nonsense and don't expect that much, if any, of your initial ideas find their way into your final design.
Once your brain has been triggered, it will work on the problem subconsciously, while you are doing other things. The next day you may find the ideas that your subconscious has been working on bubbling to the surface while you shower or just before you fall asleep at night. That's because these are moments when your conscious brain is not very busy and your subconscious brain can finally get some attention!
It is important to work on ideas in sessions with short or long breaks in-between. Your subconscious brain is where the real thinking happens and it needs time to work.