First of all, keep in mind that many people have great ideas; the real challenge is carrying them out. So by all means get excited about great ideas that flit into your head, but realize that's only the start: the hard (and even more exciting) part is yet to come.

Here are some principles of a good action or idea:

• It evokes reactions in your audience by pushing them out of their comfort zone.
• It makes you cringe. Tasteless ideas are often the best ones. Don’t reject them!
• It makes you laugh or experience a strong emotion—a sure sign it'll do that for others.
• It hasn't been done—at least not exactly like this.
• It should make a bit of sense. A bit is enough; more important is for it to make you laugh or cringe!
• Don't get bogged down in principles! Some ideas will violate all of them.

Before anything, of course, you should know what your goal is, what you want to focus on. What specifically do you want to change about the world? What do you want to fix? Think big and think small—that is, imagine the glorious future in which it's all fixed, and think of a way to get started towards that. Write it down.

The easiest way to generate ideas is to rip off actions that others have done. A couple of places to find great actions: 

• Actipedia: A joint project of the Center for Artistic Activism and the Yes Lab, Actipedia us a comprehensive, user-generated, user-ranked, and searchable database of creative activist projects. It includes new material (activist self-reports) and links also to numerous examples that have already been described elsewhere.
• Beautiful Trouble: A project of Other 98% and the Yes Lab, with dozens of collaborators (including the Yes Men), Beautiful Trouble compiles and disseminates tried and tested techniques to a wider range of practitioners via a shared database of case studies, principles, tactics, and theories.

Just browse around and see if there's something you want to repeat and improve.

For a more freeform approach to brainstorm, here are exercises for getting your juices flowing. The "Target" means the specific entity (company, etc.) you're attacking. The "Issue" is the problem that you’re addressing.

• Think of phrases and images used by your target, that you can subvert to convey your own message instead, perhaps ironically.
• What are some of your target's tactics that you could appropriate as your own? Are they known for advertising, selling, or behaving in a particular way? How can you appropriate that?
• What are the metaphors, hackneyed idioms, etc. around the issue and the target? These can be fun to play with and twist and mess up.
• What are some negative aspects of your target? How can you "accidentally" highlight them, as if you weren't very good at PR?
• What related news stories are going on right now? These can be good hooks for actions; if something is already in the news, journalists may find it easier to cover it.
• What's ironic about the situation? What are the contradictions it embodies? How can you bring these forward in a funny way?
• What cultural symbols (e.g. Tax Day, Goldilocks, Santa Claus, cash registers) relate to your target? For example, if you’re doing something around food justice, you have Ronald McDonald, the Jolly Green Giant, and numerous other corporate brands into which tremendous resources have already been poured. A symbol needn’t be only a thing; if the specific issue is government subsidies for big agribusiness, you might think about Tax Day.
• Can you take things literally that aren't supposed to be taken literally?
• For an action, what are some potential actors? What could be your stage? Who could be your audience?
• What actions have been done—around this issue, or any other—that you could copy/adapt?
• Play game in which you think of stupid, unbelievable, or inspiring headlines you've seen. Collect a list of these on a piece of paper, and select a few and try to think as a group of how to make them better, how to create even better headlines, and how to get them to pertain to the issue that you want to publicize.
• Alternately, try a game in which you come up with fictitious headlines directly related to your campaign goals—headlines that could be instructive and help promote your message. Then, figure out how to make those headlines happen.
• Again, plagiarize, plagiarize, plagiarize: think about others' actions and how you could appropriate them, or parts of them, with your particular issue in mind.
• Before settling on a final form, try pantomiming your action—running through the scenario not only in your head, but in play-acting. This can give you new ideas.