Go as low volume as possible. This will protect your hearing, and if you're using floor wedges, it'll keep stage noise from getting too loud.
Give yourself headroom. Keeping the fader at about 70% will guarantee you'll have room to turn individual instruments up in your mix later.
Ask to have instruments turned down, not up. It's always better to turn down an overly loud instrument than to boost a quiet instrument, since it's wise to maintain the same volume level in your monitor mix.
Play relevant songs during sound check. You won't get an accurate idea of how you'll fit into the mix unless you play the same genre and patches as the set.
Always be super nice to the sound guy. Say please and thank you, and never get angry when something isn't right in the monitor.
Don't worry about perfection. You're don't need your mix to sound amazing to do a great set. Get a good enough of a mix to play well, then leave it be.
Don't change your levels after sound check. This can seriously screw up your mix, especially if you boost your own volume. Often a sound guy will be forced to drop the gain on your instrument if you turn up, which will mean you'll actually end up being quieter in the monitors.
If you have a stereo mix, use it. Panning instruments left and right can help you keep things clear and out of your instrument's sonic field.
Make sure you're sounds are consistent. If you change patches frequently, make sure the levels between patches are the same, or certain instruments will "disappear" in your mix. Not everything is the mix engineer's fault.
Work out some basic hand signals with your mix engineer to adjust levels during the show. Waving your hands and pointing at various members of the band won't communicate what you mean very clearly to the mix engineer.