To help keep your shoes or trainers smelling fresh, place a fabric softener sheet in the footwear and leave overnight.
Flip flops can be kept clean and fresh by placing them on the top shelf of the dishwasher. ( Just be careful of what else is in the dishwasher at the same time !!! )
If your shoes smell really bad, through them in the washer and wash and sit to dry in the sun - this works for many types of shoes except for leather ones which may get a bit 'out of shape' after washing.
Some leathers can be washed. Just make sure that you put scrunched up paper in the shoes before drying. This helps to stop the shoes shrinking and also helps keep their shape.