1: Quit Messing With Plug-Ins.Yep, we've all been there. You're playing around with the newest compressor plug-in or the newest effect, and all of a sudden, your perfect mix sounds muddy, less defined, and over-saturated with effects. Not good!
Know when to stop with effects and plug-ins. When your mix sounds perfect to your (or your client's) ears, STOP MIXING and save a copy of the session. Then, if you wish to experiment, open a new copy of the session apart from the perfect one, and experiment away -- but make sure you've got the perfect mix preserved.
2: Watch Your Sample Rate.I was recently tracking a live concert of a charity event, and made a huge mistake: my sample rate on my interface was slaved to a piece of outboard equipment at 48kHz, while the digital feed I was getting was 44.1kHz. The result? Despite recording at what I thought was right, all of my recordings sounded slowed down upon listening to them. Resampling didn't help, and I eventually had to trash the recording and use a DAT backup, as I couldn't get my sample rates corrected well enough.
Watch your input settings -- make sure your settings are all on the same page between your various pieces of equipment. It's very hard to correct these kind of mistakes later, unless you're very skilled in post-production (and have unlimited patience, which I do not).
3: Save Your Work Frequently.My Macbook I use for recording has a really embarrassing problem: if I'm on battery power, I have about 40 minutes from a full charge until it completely dies. It doesn't go into sleep mode; it simply dies completely with around 20 minutes left on the battery meter. During a recent tracking session, I had the perfect drum sound going -- and then my Macbook died because I didn't realize my power cord came out. Needless to say, I was pretty upset because I hadn't gotten to the point where I saved my session.
With Pro Tools and many other programs, you can set your session to automatically save your work at intervals you choose. This is very wise -- you'll never miss an opportunity to save at a good junction again. Unfortunately, I didn't do that, and ruined a great session (I recovered, thankfully!).
4: Don't Use Low-Quality Monitoring.Using low-quality monitors or headphones to mix on will do you a huge disservice; you won't get an accurate picture of what you're doing, and when you get your mix done (or, so you think), and you (or your client) listens on a CD player, iPod, or car stereo? It'll sound muddy and undefined (or, the opposite: tons of high end and not enough bass).
Don't skimp on your monitoring setup. Whether you choose to go with headphones, in-ear style monitors, or traditional speakers, make sure you go with a great quality setup at the high end of what you can afford. There's a lot of great options out there for those on a budget that won't skimp on quality.