What are the most common mistakes that new engineers tend to make when recording and mixing down their tracks?
A big mistake is to stop using your ears. Always keep listening to what you create!
I've seen so many people do this, they just do things because they think they are always good things. I know somebody that always used compression because he was told to do so, even when it was completely unnecessary. When I asked him why, he just said: "I just always do this." He was clearly told by somebody else that it could (could!) be useful.
If you listen once, you decide that you need for instance a compressor and you applied it with settings that you think are appropriate, don't consider the job done. Use your ears and ask yourself if you are genuinly happy with the result.
If you record speech, don't apply an EQ to increase 3KHz and 8KHz to improve the intelligibility if you don't have a real problem with it, or if you don't hear any improvements.
Never do anything to your sound if the reason for it is not coming from your ears. Always tend to trusting your ears more than anything you learned.
Perfection is the enemy of progress. Don't spend an eternity on your first song just to realize that you don't have the time to finish the rest.
Also, for beginners it is best to finish each song before moving onto the next. This way you don't spend days recording and have nothing to show for it.
Not experimenting enough (or at all) with different mics and with mic placement. (Try the kick drum mic on the back head; try different spots on a guitar cabinet, at different distances; etc)
Having an open channel but not recording a direct in feed of the guitar. This can really save you if it turns out the amp distortion that sounded great on its own is abrasive or doesn't work in the mix.
Bass guitar. Good luck. Experiment a lot on this to find out what works, direct-in is always a good idea no matter how you try to mic it.
Really novice mistake that I made for years: using a little XLR to 1/4" adapter to run mics into a four-track tape machine, with no preamps. It worked, but everything sounded bad, buzzy, with weird high-pitched artifacts, etc etc.
Too much EQ addition: try subtractive EQ-ing first.
Too much volume: leave headroom so you have room to work with at the end of the mix for effects and mastering.
Not being aware of the limitations of your mixing environment. It's really hard to mix only in headphones and get a great mix, but it's also hard to properly align studio monitors and to acoustically treat a room. At least be aware of the limitations of your setup - "My headphones do X to the sound, but my monitor setup does Y to the sound." In a typical basement setup, your monitors and the room probably give you tons more bass than is actually in the mix - just be aware and check on different speakers, rooms, and headphones!
5 ways a newbie can be more professional:
1) Don't mess with the levels while the recording is in process as this changes the signal to noise ratio enough to make the track sound wonky.
Set your levels before you punch the record button, I generally run as many takes as needed to set the peak load.
2) Don't expect everything is in tune.
Make double sure that every instrument is tuned appropriately for the piece. I say 'appropriately' as some instruments may require a slight nuance differing from standard pitch. For example piano tunings have a wide range of deviation, compare a 'honky-tonk' piano to a concert grand.
3) Don't try to cram everyone into one track (assuming you have a multi-track system).
Be careful when you are recording multiple tracks simultaneously, assign as many channels as you need to give each voice it's own track. This will make mixing a lot easier.
4) Don't assume it will all 'fall together'.
You have to have a plan. Make a list of tracks, how they will be recorded, what effects might be employed during recording or later. In short, spell everything out before you start. Consider this your script.
5) Don't think you can remember all the details.
Make good documentation from session to mixing to master. Document what voice is on each track: guitar, violin, chorus, soprano, tenor, etc. Document anything special like "Hey that sax solo on track 2 needs to be redone because the sax needs a new reed".
Not to experiment
Thinking of the job as if there are mistakes to be made, whereas actually its an art form, and its only a matter of taste whether you like this or that (the extraordinary range of audiences and styles of recordings is the proof for this).
Not to get drunk after an impressive nights perfomance with a band, losing all the files in the morning, recovering them, and coming up with a creative solutions to fix everything.
Not to go ahead and give all those silly effects a try, then get enough of all of them and know that quality lies in the story.
Being too much insecure about yourself
Spending time reading these posts, while meanwhile you could be enjoying a smokey, jazzy environment filled with rich sounds, that you feel you want to record!
Being too stressed when things are getting slightly complicated or something doesn't work. Musicians are generally nice people (really, this is something many people forget as soon as they get behind a mixing desk!), and they allow you to claim all the time you need. As soon as you feel any time pressure during your recording session, your focus on the quality of the music and the way it sounds is decreasing. My advice in such a situation: ask your musicians to leave the studio for half an hour or so, so you can sort things out. Remember that recording and producing should be fun to do, as long as you love the music. :)
Secondly: don't be too hard on yourself. Mistakes happen all the time, and the best thing you can do is learn from them, by making them.
Failing to save your work frequently, or worse, failing to backup your work frequently (or at all).
Don't just rely on your recording software's Undo capabilities - always enable Auto-Save if it has that feature. Additionally, setup your digital workstation to automatically save a copy of your work every 15 or 30 minutes, and also save off a snapshot of the entire project including all audio files at the end of every day. Use an online backup service or flash drive. You never know when your recording software is going to hang or crash, or when the HD is going to die.
Losing a few hours worth of work is extremely frustrating, but losing an entire album is devastating.
Not doing enough preproduction.
Preproduction is what you do before you sit down in a session. This includes talking with the band, making sure they know what they plan to do. Get lyrics sheets, chord charts, tempo and key of the song, etc. The more you know about the song, the better you can figure out exactly which mics and techniques you want to use. Maybe you can borrow something that you wouldn't be able to if you found out what they were playing when you got to the session.
For professional engineers, preproduction also includes finding out how the band plans to pay you for the session. For some reason, there are people out there who seem to think that you will be happy to record them for free. If you don't plan on working for free, make sure they understand that, because it's alot easier to tell someone no before you start recording than it is after you're done mixing and they want the masters.
I think too that it is important to separate recording and mixing issues.
About recording, I have one rule I think about all the time : to avoid to postpone decision making. It was not a problem some decades ago by design, when everything is done with hardware, but it is nowadays because you can edit anything with modern DAWs, and so you can procrastinate about making good mic placements, good takes, making the band play right etc. For me, good practices in recording are about doing something right once, without even thinking about fixing it later, if possible of course. It's twice more important for someone who uses his computer to record his own music.
About mixing, I think Mike Senior from Sound on Sound has given very good advice in this article :
Not Leaving Headroom
Unless you are pushing to analog tape, you don't need to pin the meters just below the redline. Processing through plug ins will add amplitude, so you need to leave room. The best practice of pushing levels is antiquated in this world of virtual instruments and very quiet mixers.
Not using a proper audio interface (uses build-in sound adapter)
A build-in/on-board sound adapter or sound card is not made for professional audio recording. Typically it is very noisy, does not support balanced signals, cannot provide phantom power and records in too low quality.
Not recording in 24bit!
So you got the nice external audio interface with the right inputs and all, but you forgot to set up the recording format and start recording in 16 bit. You did however read the advice on leaving headroom so the signal comes in on the low side. "No problem" you think and turn it up afterwards - HISSSSSSSS! Ouch everything is so noisy now. Always record in 24 bit or higher. Always.
Ignoring technical issues
Be it noise, clipping, lag, out-of-sync, mechanical/electrical issues with instruments, bad monitoring/headphone mixes, bad buffer settings, clicks and pops, mobile phones, fridges, CRT screens near pickups, unbalanced cables near transformators etc.
Fix it, and like suggested elsewhere don't let the band stress you out.
Not getting a proper mastering done
No, that squashed L2 limiter on the master bus, or Ozone factory preset with 27-step-fx-chain will not do it. Don't get tricked by the immediate but not long lasting refreshing new and "improved" sound. Mastering is 99% about doing relative corrective adjustments to a mix and 1% about doing generally applicable tweaks like what you get from "presets".
Thinking that the mastering will fix a bad mix
True, the mastering is likely to improve the overall intelligibility and make the mix compatible with many different listening and/or reproduction scenarios. It is however not meant to be a creative and/or healing step that fixes that annoying resonance on the snare or adds ambience and other fluffy things to it all.
Make sure your mix exports has all the qualities you and the band wants, and let the mastering take care of the corrections needed to make the mix sound good in general, not just on your setup (with your ears listening).