Sit down wherever you may be and jot a list of each and every idea that comes to mind, no matter how silly the idea may be. Don't stop writing until you've filled an entire page with ideas for your essay. After you have a sufficient amount of ideas for a topic, you can start ruling out the ideas that don't work for you.
If one or another idea seems like it's missing something, merge it with another idea. The combined idea may have properties from both ideas, contributing to an essay full of well-rounded ideas.
Free writing is a writing activity that helps shake off brain cobwebs and get the creative juices flowing. The goal of free writing is to write nonstop for a set period of time (30 minutes to an hour). While free writing, it's important to not worry about editing your mistakes. The writing will be sloppy, and you'll have to shut down the inner critic in your mind urging you to stick to form. The objective of free writing is to block out the non-creative parts of your mind, letting ideas flow organically instead of forcefully.
Making memos is vital for moments when inspiration hits and you're too far from your workspace to take advantage of it. Writing down ideas as they come keeps your mind open for more ideas without worrying you'll forget the last one. If you're old school, carry around a pen and pad at all times so you can jot down ideas when they seem to magically pop into your head. If you're more in tune to the new school, download a memo or notepad application to your phone to conveniently jot and save ideas when on the go.