How to keep my hair straight after straightening it?

i straighten my hair and it is naturally extremely curly on days when it isnt so hot and i dont sweat so much it stays straight but on the days wen i sweat alot it curls at the ends or if its really humid out it just poofs up into a afro well not really but its all nasty and curly. does ne one have ne suggestions on what to do or products that will keep it straight no matter how much i sweat or how humid it gets?