Motor retention details how your motor is held inside the body tube during the flight. During the burn, the thrust from the motor goes into a thrust ring, or engine block (usually for smaller rockets), which prevents the motor from punching a hole straight through the rocket body. After burnout, the motor may be prone to slipping out of the motor mount tube. The following are methods of making sure that doesn't happen in flight.
Screw & Washer Retention
One way of holding in the motor after burnout is to attach screws to the thrust ring using tee nuts, and having large washers form a partial flange over the bottom of the motor. This method is cheap and reliable, but slightly less professional than other methods. It can be tricky to repair because thrust rings for HPR rockets tend to be permanent.
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AeroPack (COTS)
This method uses a commercial 2-piece assembly that attaches to the end of the motor mount tube. One piece is attached using high-temperature epoxy and is permanently on the motor mount tube. (There is also a screw-attached variant.) The other piece screws onto the end of the assembly, and has a small flange that does not interfere with the motor exhaust gases and holds the motor in. This assembly serves as a thrust ring to the motor itself, although more structure is usually required to transmit the load to the remainder of the rocket. This solution is easy to implement and reusable for many flights.
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Bolt/Threaded Rod
Some motors come with a forward enclosure which has a spot for a threaded rod or bolt, which can be attached to a bulkhead to hold in the motor. This does not replace a thrust ring. This method was used for Project Odyssey and Therion.
PICTURES OF THERION IMPLEMENTATION
Upper Ring (For adapter)
For smaller motors (no more than 54mm) which use adapters, the motor needs to be retained within the adapter, and the adapter needs to be retained within the motor mount tube. If the motor case extends significantly beyond the length of the adapter (at least 1.5"), then a ring of painter's tape can be wrapped around the motor casing just above the adapter such that there is no travel distance between the bottom of the motor and the bottom of the adapter if both are held by the adapter alone. The ring of tape needs at least 5 layers - it should not go inside the adapter and friction-lock the motor - it should seat against the top of the adapter.
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