if your mixing for hours genre changes make it easy, start of with down tempo, move up to disco, then to house then to electro, then to trance then to dubstep or hardcore then reverse it and go the other way...so you have a looooong rise and then the same long fall but if your doing mini sets it gets a little harder, for me at least more bass spells more energy so i keep the dirty as bass line's and wobbles n stuff for the middle of the mix, otherwise if you start with this massive bassy song its hard to go up from there... get me?