The look on their face when you get loose and crush their throat? Priceless.
Reverse the belt so that the side that will be facing your body when being worn faces you. Cut a small strip of tape to cover the key.
Affix the key to the belt.
Using four more small pieces of tape, tape as shown to give the most flexibility to the belt and reduce the chance that the key will become dislodged.
Other small tools, such as an Xacto blade can be attached similarly. This will be handy to deal with nylon cuffs. If you do this, tape the blade vertically with the tip pointed down towards your buttocks. When you reach inside and grab the tape, it will form a small handle and allow you to grip it better. You also won't stab yourself pushing your finger down between your belt and pants.
Another belt to use is one made of bonded leather. Typically, the inside of the belt is split hide and has a vinyl surface stamped to look like leather. You can separate the two layers and make a small pocket.
Some full leather belts are made of two layers of leather sewn together. Simply cut the top threads to make a pocket. Make sure you seal off both sides of the pocket or your tools may migrate and be impossible to retrieve.
It is a very serious concern of mine that some of us are going to get grabbed off the street with the passage of the 2012 NDAA and The Enemy Expatriation Act. I think it is important that you always have some means with you to assist in escape and evasion.
I normally use black tape, but in the images I used silver so it would show up. Look for gaffers tape if you are able to. Duct tape doesn't always deal with water well, while gaffers tape is water proof. Important if you expect perspiration or live in a humid environment.
There are formulations that withstand high heat. Read the packaging to determine if it meets your particular regional requirements.
Gaffers tape can be found online, professional music stores, at places that sell theatrical supplies and even at Staples. Expect to pay more for it than duct tape.
Handcuff keys can be purchased online and at gun shows. They're usually around a buck a piece.
Ladies, wear a charm bracelet? Maybe you should add another "charm".