Being chosen as the opening band for a more established act can mean great things in your music career. You will get to play for larger audience than you might draw at your own show - an audience who might then get excited about turning out for your next headlining set. An added bonus is that peppered in amongst those potential new fans might be members of the press and industry who may become contacts for future opportunities.
All of those good things could evaporate pretty quickly if you violate some of the unwritten rules of being the opening act, though. Some of these rules might a touch frustrating, but take the long term view - making a good impression now is money in the bank for your future music career.
There may not be a formal arrangment for you to roll up your sleeves and help promote the show, but get on board and do what you can. Announce the show on your website, social networking sites and via your mailing list. Be sure to include info about the headliners in the promotion you do to your existing fans.
Contacting the local press and radio may also be helpful, but consider checking with the show promoter before you do that. They may have plans for reaching out to the local media, and you don't want to step on their toes and confuse the message. Generally speaking, the larger the show, the larger promotion machine behind it, so do check before making the media calls.
2. Watch The Clock
When the headlining musicians, their management, agent or the show promoter asks you to be somewhere at a certain time, be there. Yes, even if you know if absolutely everyone else involved in the show is going to be late and you're going to be spending a lot of time standing around waiting. If something happens that is going to delay you - getting lost on the way to venue, flat tire, forgotten instrument, etc, etc, etc - call someone and let them know. Even if they treat you like you're giving them T.M.I., better to err on the side of being thorough and showing that you respect the schedule set for you than to bank on the fact that everyone will be cool with you rolling in when you can.
3. Accept The Soundcheck
In most cases, soundcheck starts with the headliners and finishes with the first opening act. The reason for that is partially a practical one - the first opener will take the stage first, of course, so when they soundcheck last, the stage is set up with their gear so the show is ready to start.
However, the reason is also partially hierarchy. Allowing the headliners to get the first crack at soundcheck means they can kind of take their time and soundcheck until they feel good about their set. Sometimes, this means the headliners end up taking up ALL the soundcheck time - or most of it - and that of course means the opening act gets little or no time to check their own sound and get comfortable with the stage/acoustics.
For an opener, that can cause some serious stress, but your best bet is to grin and bear it rather than kicking up a fuss. Sure, it would be great if the headliners made sure everyone got a pop at a soundcheck, but it IS their show and their perogative to take the time.
4. Discuss Merch
Before you assume that you'll be setting up a merch table the night of the show, discuss it with whoever booked you for the gig. Sometimes, headliners (or their reps) frown on support bands selling their merch because any money thrown your way is money not spent on the headliners' merch. That may rub the wrong way - especially if the headliners are making big bucks for the show while you're getting a pittance - but you're kind of bound to the rules set by the people who invited you to play the show. Have a discussion about this before the night of the show.
5. Respect The Set Length
Even if it feels like the audience is eating it up and you're having a great time on stage, wrap up your set when you're supposed to. When you run over, you take time away from the headliners. It's important that they get their full set - or if they don't, that it is not your fault. Remember, the headliners are who the audience is REALLY there to see, so just be glad you made some new fans and promise them a longer set in the future.
6. Stay for The Show
Unless there is a valid reason why you have to play and dash - you've got a plane to catch, a 14 hour drive home, an illness or something along those lines - don't skip out before the headliners play their set. Yes, even if they are not your favorite band, stick around and watch them play.
7. Say Thank You
Say a quick "thank you" to everyone who helped you land this opportunity and everyone who helped the show run smoothly. From the headliners and their reps to the venue manager and sound engineer, a quick thank you goes a long way.