Practice the literature so that you always play it right.
If you can't, practice more until you can.

Prepare your gear.
Clean your instrument, apply fresh rosin, make sure your strings are in good shape, bow is working properly, etc.

Prepare your music binder with all music in the correct order.
• Hole punch and/or tape pages for quick turns.
• Make sure there is nothing loose that can fall out.
• Carry extra music (from previous performances) in your backpack, just in case it is needed.

Make a written checklist of EVERYTHING you need to perform and use it EVERY time.
This includes (but may not be limited to...) instrument, bow, shoulder rest/rock stop, music, pencils (at least 3 sharpened with good erasers), tuner, rosin, spare set of strings (old ones that are still in reasonably good condition make great backups in an emergency), music stand, stand light, extension cord/spare batteries, etc.

Dress appropriately.
Typically concert black (including shoes) unless another specific dress code is provided.

Know exactly where you are going.
Includes: what address, what route, what building, what venue/room, which door to enter, where to unpack, etc.

Arrive EARLY!
• "To be early is to be on time; to be on time is to be late."
• Better to travel across town and wait (grab a meal or a snack) rather than be caught in a traffic jam that makes you late.
• Give yourself plenty of time to unpack, tune, warmup, find your assigned seat, acclimate to your surroundings, check for any last minute changes/program notes, and visit with colleagues and friends.
• Be in your seat no less than 10 minutes prior to your performance time.

Be aware of other people at the venue and maintain a warm and friendly disposition towards them.
This includes fellow group members, conductor, sponsor/coordinator, emcee, guests, etc.

Be aware that you are always being observed by others.
You may not be able to see everyone who is watching. Your behavior should ALWAYS reflect positively on you, the group, and the school you represent. 

When performing, be focused and engaged.
Maintain your composure at all times. Absolutely NO goofing around or allowing yourself to get out of control (talking, laughing, crying, other emotional responses).

EXTRA but just as important as the previous ten: When you arrive at the venue, help with setting up. Afterwards, help with tearing down until everyone is ready to depart at the same time.
There are no "roadies" to do the work and it should not be left to the director to pick up the pieces. You may have family or friends waiting, but so does everyone else. Look for what needs to be done. If you don't know what to do, ask. No task is too menial (including resetting a classroom, picking up garbage, or cleaning up a rest room).