It can be difficult to stay focused when you hearing the same track for the tenth (or hundredth) time in a day. By keeping a few clear objective goals in mind, you can avoid overlooking some important aspects of the track. Even the simple things like whether or not you can understand the lyrics, or if the different parts of the mix can be heard clearly enough. It also doesn’t hurt to give it a rest for a day before coming back to hear the track with fresh ears.
You can read more tips here: http://audioundone.com/20_mixing_questions