Make sure that all sound sources sound as good as they possibly can before you record them. This means checking the intonation of all stringed instruments and tuning them before every session. Similarly, replace old drum heads and make sure the drums are well tuned before you put a microphone on any of them. If your instruments sound good acoustically, they'll be much easier to record well. Making up for poor instrument tone by experimenting with equalization (EQ) controls and special effects devices is an inefficient and time-consuming way to approach a recording session.
In the old days of analog recording, an occasional spike in volume that sent the recording level meter into the red was no big deal. In fact, a little tape saturation was all part and parcel of the appeal of recording to tape, since the resulting distortion was perceived to add warmth and musical character. This is certainly not the case with digital recording. Any part of an audio signal that exceeds 0dB is literally cut off, or "clipped." This amounts to missing bits of information, resulting in an unpleasant and decidedly unmusical digital distortion. To avoid this, make sure that when you set the recording level of your tracks, the peaks are somewhere between -12dB and -6dB. This will ensure that every bit of digital information you send to your recording device actually gets recorded, and nothing gets lost, or clipped.
Although it can be tempting to record tracks with effects (a wet signal), this can limit your options when it comes to mixing the song. Once a part is recorded with reverb or an echo-type effect such as delay, you're stuck with them, so recording without effects (a dry signal) is the way to go. If you're recording a singer who feels that a reverb-soaked track helps him deliver a more inspired take, you can always add it to his personal monitor mix so that he can hear the reverb effect in his headphones, while you record the signal dry.
As a sound engineer, you'll likely have a whole range of software and hardware EQ devices at your disposal. While it can be tempting to view them as mixing tools that you can use to repair poorly recorded tracks, you'll be better served by working harder during the recording session to get the highest quality sound you can. This could mean experimenting with different microphone types and their placement in relation to the sound source, or finding the most compatible guitar amplifier for the lead guitar player. If you put in this work upfront, then you'll be able to use your EQ controls sparingly as a way of perfecting your recorded tracks, rather than trying to rescue them.
A hard drive failure can be catastrophic for both the engineer and client. Hours of hard work and inspiration can be lost forever, costing you time, money, and even your reputation. It's imperative that you back up audio files after each session. Ideally, you should have a dedicated external hard drive to record all of your files to, and another hard drive to make backups. Make this a part of your routine, and you'll be covered in the event of a computer crash.