How much headroom should a mix have? Avoid clipping

Leave about 6 dB of headroom: turn your tracks down so the master peaks around -6 dBFS. Above 0 dBFS there's no room left and the peaks get chopped off, which sounds harsh and distorted. Make the mix sound good at a sensible level, then make it loud at the very end, in mastering.

How to do it

1. Pull every fader down, or turn down the gain at the start of each track.
2. Build the mix so the master peaks between -6 and -3 dBFS.
3. Keep the master fader at 0 and turn down tracks, not the master.
4. Don't put a limiter on the master while mixing unless you're only checking.
5. Bring the level up in mastering with a limiter.

Why it works

Clipping cuts the tops off the waveform and adds harsh distortion. Plugins also behave better at sensible levels, and the mastering engineer (or you, later) needs room to work.