For patches I usually put the snake in a water soaked pillow case for a few hrs and they usually work it off. I leave the pillow case in the cage while they are doing this.
For larger areas and difficult sheds I put them in a tub of luke warm water that. In these cases I've left them in for up to a day and when this is the case I'll put a portion of the tub over heat tape so they have a warm spot. The water should only go halfway up their body.
Lastly, if it's only a few small spots and they are tame just do it by hand under a faucet with light running luke warm water. A pair of gardening gloves with those little rubber gripping beads helps too. As they move through your fingers the beads catch the skin and it slides right off. It's less stressful for the snake because you are not picking at them.