Yeah, it`s red for a reason, it means stop.
Keep your tunes out of the red, people can hear em, all your probably doing is cutting in the limiter on the rig, and making the tunes sound shite, as well as damaging the mixer.

A good engineer will have set the levels way below the red anyway, so keep em there.

With spinbacks and scratching, back of the bass a little.

Take utmost care on your track levels. The bigger the rig, the more pronounced small differences in levels become. Go by ear and also by the pfl meter.

When you first come onto a rig, especially in a loud environment.
Turn the headphone output right down. Plug yourself in.
Wait 10 seconds or so, and then slowly bring the volume up to a comfortable level. your ears will love you for it, and you`ll find mixing much easier.
Some DJ`s just rip the volume up, and pound their ears to submission.

Never pump the Bass beyond flat on the EQ, unless you know the headroom you are dealing with, if in doubt, talk to the engineer.
Who will probably sneer at you and tell you not to do it, but it`s better to ask.

Monitor levels. If you can set the booth monitor level, do the same as the headphone trick. Stick your headphones on, turn down the booth level. Set your headphone level, then bring up the booth monitor to match.

Your monitoring level, how clear and comfortable it is, is probably the most important thing to ensure you get good control over your mix. It is the lifeline to the rig, it may seem obvious, but make sure you get everything as comfortable as possible.
It`s far more important than slappin a tune on as quick as possible.

Filters and effects.
Again be careful with the levels on these. Nothing worse than some DJ wackin the res right up on a high pass and caining the hell out of the tops on the rig.
Flange is also one to be careful with.