Copier toner is a powder containing pigments and resins. Toner melts when heated, which is how it bonds firmly to paper during the printing process. When toner gets on clothing (which can happen easily), it won't form an indelible bond immediately. The sooner you treat it, therefore, the better. However, there are definitely right ways and wrong ways of treating this kind of stain. For example, unlike treating other types of spills, the initial steps for treating copier toner on clothing should not include water.
Remove the article of clothing as soon as possible. The sooner you attend to this, the less likely the toner will spread all over or settle into the fabric. Shake it vigorously to get as much of the loose toner out as possible.
Use a vacuum attachment to vacuum loose particles out of the clothing. The more toner powder you can get out via dry methods, the less likely the stain will set.
Fill a sink basin with cold or tepid water. Immerse the item in the water and agitate the clothing to remove more of the toner. Drain the sink and then run cold water again over the item to rinse it.
Wash the article of clothing in a washing machine. Use cold water (only) with regular, mild detergent.
Air dry the article in the shade or indoors. You must not use a dryer or dry the item in the sun in case any toner particles are left--if any remain, the stain can get set if the clothing gets hot. It will be then be much harder, if not impossible, to remove.