Your Personal Search Engine is your brain. Particularly, the way your brain always gives you something back if you put something in.
If you hear the word "camel", you will think of something. Perhaps you will visualize a camel, or remember a children's story about a camel that you liked when you were a kid, or be reminded of the desert, or think of the funny way that computer programmers run words together. Or all of the above.
It's as if your brain is like Google. Whatever you "type in", all sorts of surprising things associated with it come back.
Doing an improv scene is little more than letting your scene partners type things into Your Personal Search Engine and then adding to the scene whatever your brain gives back. Pretty much everything triggered by the action of the scene will be appropriate to the scene and funny. Funny because it makes so much sense in that hard-to-define way.
There are just a few exceptions. If you put "funny" into Your Personal Search Engine, you'll think of things that were funny in other situations. Perhaps jokes you heard a long time ago, or forced attempts to be funny. (See Gagging.) The funny thing is that telling yourself "be funny right now" brings up nothing that's funny right now.
Same with "good" or "clever" or "original". If you tell yourself "be good" or "be clever", you'll get dull or forced stuff. That's the Paradox Of Improv. If you tell yourself "be original", you won't be original, you'll just be contrary.
To be funny, good, clever, and original is actually very easy, though. It's automatic! Just Listen, feed Your Personal Search Engine with the specifics of the scene in progress, and take whatever comes out. As long as you're allowing the scene to drive you instead of consciously trying to be funny or clever, nearly everything you say will be hilarious and brilliant. People will wonder "How did you come up with that? It was so perfect!"
It will seem brilliant to them because no two people's Search Engines are alike. Your Personal Search Engine doesn't reflect cultural stereotypes or standard gags, it reflects the unique workings of your brain and your own unique life experience.
If you give Your Personal Search Engine a phrase that includes the word "not", the results you get back are exactly the same as if you leave "not" out. For example, if you tell yourself "not a bear", you'll probably think first of a brown bear, then of Gentle Ben, then of a polar bear rearing up on its hind legs, and so on.
If there's something that you want to avoid doing, you can get Your Personal Search Engine to help you out by giving it something to search for rather than something to not find.
To get away from something you're stuck on, try Double Word Association.
See also: Add Information.