When I mix my songs I usally wait for like a couple hours after im done making a beat, sometimes ill just wait till the next day, so ill have fresh ears to mix on instead of tiring them out. Something else you can try is try mixing the high end first on your track, like around 5k on up, those are the frequencies I mix first, because when I started mixing those were the frequencies that made my ears hurt and gave me a headache. But I say the best thing to do is get some good monitors like safeandsound suggested. You can use headphones to mix too, but you still will want to mix from monitors. And try setting a day aside just for mixing, so you wont put that much strain on those ears. ~1~