1. Always set out your goal in the beginning of the mix session. Listen to the rough mix, communicate with the client/artist about their vision for the song, ask for references whenever possible. This will help you establish a clear sonic idea to go for right from the get go. It put things in perspective.
2. Don't start until you FEEL it. There have been times when i had songs to mix within a small time window and those are certainly an exception, but generally you don't want to start putting up faders just for the sake of starting. The reason is simple. We work off of creative ideas and energy, and so the best time to start is usually the moment when we feel creative ideas flowing in such a way that it INSPIRES you to start mixing.
3. Good balance is key. I always try not to reach for EQ or compressors or other tools too quickly into the mix. Your faders can act like an EQ most of the time. If you want less low end in the mix, pull down the things that has low end in them. If two elements are clashing arrangement-wise and/or frequency-wise, see if you can pan them in opposite direction before you start EQ-ing them.
4. This one has been said many times by many people but it is absolutely right and is really important for us Mix engineers. Take breaks regularly. Remember that our ears get tired quickly and we most likely start to lose perspective after a couple hours. We can be stubborn at times and think that we can still go on, especially if we're in the middle of trying to fix an "issue" within the mix. Do yourself a favor, let it go for a moment. Take a break and you can go back into it after half an hour.
5. Lastly, i find that it always works in your favor to try to "master" your mix once you're finished (or close to finish) in the end. You dont have to actually master it, just try putting a limiter as the last chain on your master bus and crank it hard. Try to make your mix sound as loud as other mastered songs (perhaps your reference tracks) and see how it holds up. Often, it will reveal bad things in your mix and now you get to fix it before it goes to the client for a preview. It's always better to fix mix issues in the mixing process rather than relying on the Mastering engineer afterwards. Once you corrected the mistakes (if any), take out the limiter or tune it down a little so that your mix is now ready for preview.