A good recording starts with the sound source. If an instrument's tone is poor, a good recording will only serve to further illustrate its shortcomings. Making sure your instruments sound good acoustically is the best way to make it sound good when amplified. Guitars should be restrung; their intonation should be checked, as should the fretboard to see if any fretwork needs to be performed. This will help resolve string buzzes and tuning issues that can spoil a performance. Drum hardware should be checked to eliminate annoying rattles; new drum heads fitted to improve the tone of the drums. Don't forget, too, that crackles and pops during a performance can cast a dark shadow on any recording session--check all instrument and mic cables first to make sure they're in good working order.
When all instruments are in good working order, the next step is to make sure they're all perfectly in tune. Badly tuned drums will sound bad no matter how you mix them; out of tune guitars will sound amateurish no matter how well arranged and played your song. Tune up before you start recording, and check it often during the session.
Do everything you can to avoid "clipping" when recording. Give yourself a little headroom by using your pre-fader input level (trim) control, so that the peaks of your signal don't exceed -3Db. Whenever the signal exceeds 0Db (known as unity gain), the signal's waveform will be clipped and digital distortion is the result, so avoid it at all costs.
Forget the old adage, "We'll fix it in the mix." You're better off working on getting the best sound you possibly can during tracking. Leaving yourself too much to do at mixdown can be a headache. The fact is, not everything can be fixed in the mix. If your tracks sound good without gobs of EQ and without being saturated with effects processors, then your job at mixdown will be so much easier. You can then spend a little time tweaking and improving tracks that already sound good, rather than trying to work miracles on poor-sounding ones. It's also a good idea to limit the number of takes of each instrument or vocal tracks. If you have five or six different lead vocal takes to edit and assemble when you're mixing, it can get awfully tedious. It's better to work on getting one or maybe two stellar performances you can commit to, so that the mixdown session is a simpler, more streamlined process.
When mixing, be as critical as you can. Listen carefully to all of the tracks you've recorded and ask yourself if they're necessary. Forget about how long you spent working on arranging and recording them; if they're simply taking up valuable headroom and not improving the song, take them out.
It's also important to take frequent breaks to give your ears a rest when working on a mix. Ear fatigue can cloud your judgment and lead to bad decisions that you'll need to fix later. It also pays to listen to rough mixes on different playback systems. If the mix sounds good on your home stereo, a boombox and in your car then you know you're close to a final mix.
Play rough mixes of your music productions for different people and ask for their opinion. While asking fellow recordists is a good idea on a technical level, it's also worthwhile asking casual listeners for their thoughts. Sometimes, an untrained ear can alert you to obvious shortcomings you may have overlooked because you're focusing on fine tuning and missing the big picture. After all, you are producing music for the general public, rather than fellow engineers, right?