1. Its a special previlege and an honor to be asked to give your objectivity when dealing with someone elses work.
2. Listen to the song and i mean really listen to the song not just hear it and start turning knobs.
3. Always go with your gut instinct when making decisions in the mix.
4. Remember that its not your song and the client will have to live with it once its out of your hands.
5. Lastly and most importantly...mixing a song is supposed to be fun not just work.
That's why you got into in the first place.