Use products that promise humidity protection
The hair at your roots is usually the first to revert back to curly because it absorbs the most moisture whenever you sweat, take a hot shower, or go outside in humid weather. Back in the day, all we had was grease, but now hair care companies make "humidity protection" and "anti-frizz" products that prevent reversion by covering your hair with a lightweight protective film. Before you straighten your hair, prep it by shampooing, and deep conditioning. Then apply a leave-in conditioner and a styling cream or foam that promises to "block humidity" or keep your hair "frizz-free".
Choose a "flexible" hold hair spray or skip it
A good hair spray can lock your hair in place better than any other product, but you don't want it to be too stiff or sticky. Choose a hair spray that says "flexible", "workable", or "natural" hold like Nexxus Comb Thru. The best hair sprays contain alcohol denat., which is usually a no-no for ethnic hair. To avoid damage, hold the can 10 to 12 inches away as you spray, which allows the alcohol to evaporate before it hits your strands. To get the most days out of your straight hair as possible, skip hair spray altogether.
Protect your hair from shower steam using a moisture-wicking headband and a shower cap
You'll definitely wear a shower cap in the shower, but your hair may still revert from the steam. Keep your hairline away from the water by putting on a moisture-wicking headband like the ones sold by Nike and Under Armour. The microfiber in the headband will absorb moisture and trap it between the threads of fabric.
Wrap or pin curl your hair at night and tie on a scarf
Your straight hair will last longer if your strands aren't allowed to rub up against each other while you sleep. If your roots usually revert quickly, take the time to pin curl your hair at night after applying a very small amount of moisturizer or serum to each section. Tie on a satin or silk scarf to keep your hair in place and your edges smooth. You can get away with less bedtime prep, but the better your hair is protected at night, the better it will look the next day.
Following these tips can make your straight hair last for weeks, but that's a long time to go without washing your hair. If your scalp starts to get itchy, it is time to shampoo and condition. If you can't wash right away, blot the oil build up on your scalp with an alcohol-free toner. To keep your hair healthy with minimal breakage, it's best to shampoo and condition at least once a week.