Write for five minutes about an activity, sport, etc. that you enjoy. Make sure that you have a main idea, supporting ideas, examples, and correct grammar.
Now, write for another five minutes about a different activity, again, one you enjoy. This time, write as quickly as possible without regard for proper grammar, punctuation, "format," or even making sense. The goal is to get as many ideas out as you can by writing whatever pops into your mind and going from there.