After a half hour, wash out excess, but don’t wet hair first: Mix shampoo with a splash of water, saturate strands, then rinse. Without a lot of extra H2O, shampoo can break down oil better so it rinses out easier. Head off tangles with only a little … But temperature is actually less of a factor than duration, says David Herschthal, M.D., a dermatologist in Boca Raton, Florida. So you can warm up a chilly weekend with a steamy stream, but limit it to five minutes. …