When mixing down, play back and listen to your master version through different speakers. If it still sounds good on cheap speakers, it passes the test. If possible, mix down on a later day to the recording. Have a break, as fresh ears hear things differently.
Similarly, do not mix on your own - someone else might point out things you have missed. And do not keep turning the volume up to hear things better. Keep an eye on the volume and, if necessary, turn everything down and have a break. It will seem loud enough when you come back.
Above all, always keep in mind the overall sound. Do not listen to one instrument at the expense of others. Walk around the room while listening to a playback. Remember: sound is flexible and can be changed by the slightest factor, so use your ears.