If you spend more than five seconds in London, you are bound to notice the omnipresence of the following five words: Keep Calm and Carry On.
The origin of the phrase and the associated red poster is from 1939, when the government's ministry of information distributed many leaflets with propaganda encouraging positive tones before the expected blitz. This phrase was one of them, along with two others which haven't exactly resurfaced to the same degree of fame: "Freedom Is In Peril: Defend It With All Your Might" and "Your Courage, Your Cheerfulness, Your Resolution Will Bring Us Victory" (thanks wikipedia).
The leaflets were never displayed publicly during the war, and apparently their distribution was quite limited. Which explains in part why their existence was all but forgotten for over sixty years.
In 2000, when a bookstore owner re-discovered a peculiar red Keep Calm and Carry On poster in a box of old books, he found it unique and interesting enough to hang it prominently in his store. It sparked immediate interest, which grew dramatically, and before long copies of it were available for sale all over the UK.
Britons are quite proud of their country's resolve in the war, and the uncovering of this kind of war relic fed into that pride and nostalgia.
Today in London, you can find the iconic font and layout just about everywhere. But I'd guess that the "Carry On" part only makes it into 20-30% of merchandise.
Once the poster reached its tipping point and gained fame, rampant commercialization took over and all but destroyed the essence of the message. Merchandisers have produced every conceivable spinoff of "Keep Calm and Something Something" to the point of absolute banality.
Here are actual examples of t-shirts I've seen people wear in the last several days:
Keep Calm and Gangnam Style
Keep Calm and Listen to Trance (and I thought, are these even compatible?)
Keep Calm and Enjoy Croatia... That's right, we were IN CROATIA and apparently these were being sold
My take is that if you're going to buy or create any merchandise that bastardizes the original phrase, it had better be funny or clever. And as you can see by the examples I've noted, not many fit the bill. This one pretty much sums it up:
Otherwise if you want to get in on the craze, just stick with the original.