Using Anal Plugs:
Your collection of anal plugs should be at least three sizes: small, medium, and large (these sizes should be based on the slave, not what's available on the market). Obviously, you should start with the smallest size and make it comfortable for her.
The slave must learn to keep the plugs inside and move without them slipping out. Lubrication is necessary, but not excessively, otherwise it will be impossible to move and keep it inside, especially if it's small.
If you've used the proper lubricant, the slave should be reprimanded and punished if the plug slips out. Move up to the next size if muscle weakness seems impossible to overcome.
You should require your slave to wear the plugs for increasingly longer periods of time until you're sure she gets used to the plug. At that point, move to a larger plug and start the process again.
A slave wearing a butt plug is rarely comfortable enough to "forget" she's wearing it (if she does, it's time to upgrade). The constant awareness, the feeling of being open, and the knowledge that someone is controlling how she performs her duties is an effective method of keeping the slave in "slavespace"; it will make her feel powerless in her position, and a feeling of subordination is practically guaranteed.
From original site "BDSM Canarias" [BDSM Canary Islands], this is a portion of the article “Manual de Adiestramiento" [Training Manual] URL http://www.bdsmcanarias.es/textos/Manual_de_Adiestramiento.htm