Be prepared to foil pick-pockets with a decoy wallet. Having anything stolen from you is saddening - unless you make something specifically designed to be stolen! Note that this is not for use if you are being robbed!

Step 1: Supplies

You need some sort of a wallet. The one I have pictured was a gift from my brother and I honestly would be sad about losing it. I'll make a replacement decoy or keep an eye out for a cheap one to use.
A singing card. This isn't critical but sure does make the project more fun and the decoy much more enjoyable to show friends.
Low-value currency.
Business cards, expired gift cards, or other junk to thicken up the wallet.
Anything else you would like to stuff in there to further prank the thief. I used some relevant cards from a game of Apples to Apples, some cards from a deck, and a page from my daily bacon calendar.

You may also need scissors, glue, a translation book, and a pen.

Step 2: Some Money

Of course, this step is completely optional. You might not want to give the thief any money at all! Pictured is one USD which is of almost no use in the places I usually travel (bills must be over $20 and totally crisp in order to be exchanged), 23 cents worth of Chinese currency, and about 20 cents worth of Mongolian currency. It is just the right mixture that the recipient will surely know that the wallet was intended to be lifted.

Step 3: Filler

I added some playing cards, business cards (some of use to me but duplicates of ones that I really need), and relevant cards from Apples to Apples that describe the wallet. Having the wallet thick enough to be noticed is important. You want the thief to focus on it instead of your bag, your friend's thinner wallet, or the bills and cards you have tucked elsewhere. This thing should look like the mother-lode!

Step 4: Useless Cards

Save the fake credit cards from junk mail, membership cards you get once for the 10% discount, SIM card packaging, dead phone cards, maybe even expired credit cards if you want the thief wasting time trying to use them.

Step 5: Make Your Wallet Play a Song for Them

I would love to see someone wire a piezo alarm to their wallet but just using a cheap card will likely give the thief a real surprise. Just cut out a piece of a singing card that will fit into the wallet. A bifold wallet works quite easily - you just need the piece to fold just like the wallet does. Measure or trace the wallet, cut out and insert it. Try not to wear the battery out with your excitement over this delightfully simple addition.

Step 6: Leave Them a Message

I cut out some more pieces of the card and used my multi-language phrase book to leave the thief a message. I tucked the message in with the cash since they are likely to grab it even though it is nearly worthless. I doubt they would go through the bogus cards too long!