Keep your feet dry. Before you slip into any plastic shoes, even ones with lots of exposed skin and ventilation, make sure your feet are clean and dry. Wash them with soap and water and rub them vigorously with a towel. Moisturize them lightly, including the tops and in between the toes. Sprinkle baby powder over your whole foot and rub it gently into the skin, making sure to pay extra attention to the spaces between the toes.
Keep your shoes dry. Plastic shoes aren't very breathable; that's why they're so cute in open and lattice styles, but it also means they trap heat and moisture, which leads to rubbing and blistering. Wipe your shoes down with a damp cloth and then dry them thoroughly. Sprinkle baby powder inside the soles and shake your shoes back and forth slowly to spread the powder evenly.
Apply a protective balm to your skin. Slip into your shoes and note where your toes rub against the plastic. Roll a protective runner's balm over the areas of your toes that are prone to rubbing. Let the balm set or dry as needed. This may be enough protection for your skin if you're wearing flip-flops or other open-toed plastic sandals, but jellies or closed-toe shoes may require an additional step.
Apply blister dressings. If it gets really hot out or your shoes just aren't letting in enough air to combat friction, pull out the big guns and apply blister dressings to prevent painful rubbing. Snip the dressings to exactly the size of where your shoes rub. You don't want the dressings to show! Stretch them over the hotspots and press down gently until you know nothing will slip.