When the straightening is finished, mist your hair with hair spray, gloss sheen, a/o other styling products. If it's a misty or humid day outside your hair will instantly puff up and frizz as soon as you step outside -- this is why you need to make sure you're straightening your hair on a day that is pretty dry and sunny.
Your straight hair will last until you take a shower and wash your hair. If you're lucky, the hair will be left in the exact same condition as you started out with. If you're not so lucky, your hair will be left looking like you stuck a finger in an electric socket until you go to the hairdresser and get a deep conditioning treatment.