The Art Of Gain Staging
When mixing music, gain staging is an essential tool in preserving headroom. Gain staging refers to the process by which you allocate dB levels to individual tracks, such that the final mix bus has a healthy amount of headroom.
A great analogy to understand this, provided by mixedbymarcmozart.com, is with a bank account. The balance in your bank account, analogous to the level of dB in your master bus, is an indicator of how much you can afford to spend, or the amount of dB you can spend on individual tracks.
In summary, watch the level of your faders and be cognizant of when your master bus is too hot. For more info regarding gain staging, go here.