Here's my Top Five tips. Feel free to add your own in the comments.

1. Avoid stuff that's been done. True of any medium, of course. But with Ambient - when you have the whole world for your canvas - it would be a shame to do an ad in washrooms, on coins, or on escalators... when these have already been used so many times.

2. Don't expect it to actually happen. You need an anti-rejection suit to survive in this business at the best of times. But with Ambient ideas, the attrition rate is even higher than for other media. TV spots are booked. Press schedules are booked. No one ever 'booked' an Ambient ad, therefore it doesn't need to happen... the Traffic people don't have to push for it to happen, and the Client doesn't have to sign off on anything they don't absolutely love. So manage your expectations accordingly.

3. Make the location fit the idea. An ad is not more exciting just because it's on a crane or a toilet. It's exciting if it's relevant.

4. How to judge them. My criterion is very simple - 'Would It Get In The Metro?' (London's free morning newspaper). Every morning, they run a couple of funny or weird pictures, and quite often these are advertising stunts. This is free PR for the brand, and gets seen by many more people than see the physical Ambient piece itself. If you think your idea is funny or weird enough to get in the Metro, draw it up. Otherwise, don't.

5. Do it yourself. As I already said in Point 2, Ambient ideas almost never happen. So why not do it yourself? You only need one location. You can often use cheap materials, and take the photo yourself. Then if nothing else, you'll have a nice piece for your book.

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