Try not to wait until the dust bag is full. Most vacuums decrease suction power as their dust bag fills (this is even more important if you own a Fan Forced type design). Also, the dust bag acts as one of the stages of filtration .The bag's pores allow the bag to breathe and create air flow so you don't want them to get blocked. This is another reason not to reuse a dust bag, which will also stress your motor system.