The best is not to try and focus on any particular frequency or sounds at first. I am a mastering engineer, and the best technique I use is to almost not focus too much initially. It sounds counter intuitive, but this approach allows you to get an immediate first impression on the overall track, especially if it is one you know.
I'm no mastering engineer but I've also found this is the approach that works best for me. It seems listening without focus highlights what's wrong (or right) rather quickly, at times it's immediately apparent.