I've played countless "local" gigs with a 4 band bill where everyone had an equal draw, many "opening" gigs where the headliner clearly brought the majority of the room and I've headlined (both locally and on tour) where I clearly brought the majority of the room.
When you're just getting started in your local market most gigs won't be technically "headlining" or "opening" (no matter what number you are on the bill) because most local bands typically start off on a level playing field. However, some bands break out and start to draw big numbers.
It may feel like once you are officially anointed as a "headliner" for a show (or tour) that you have made it. You haven't. Even if you got a hit single on the radio (anyone want to ask how the "Had a Bad Day" guy is doing now?). Success in the music industry is fleeting. The worst thing you can do is act like a headliner and treat the openers as unworthy pissants.
If you're headlining in your local town (and actually headlining - not just playing last) it's your job (if the talent buyer doesn't do this) to tell the opener(s) their set time/length and ask them to stick to it (most of them, remember, will not have read The Opener). You have earned rights to the best merch location, but definitely leave room for all other bands to setup their stuff as well. If you're running the night you'll want to make sure that the openers know their load in/sound check times and know what the drink/food deal is. Don't expect them to know this info. Give them an allotted sound check time too - to make sure everyone gets enough time to check. Be courteous of their time and don't run your sound check over time either. If you're paying them a flat amount, come to the venue with that amount of cash on hand just in case the venue pays you in a check (and you don't make enough merch cash to pay out the openers). There's nothing worse than having to track down a check - especially from fellow musicians. If you're giving them a cut of the door then make sure you get a rep from each band to settle up with the venue at the end of the night so there's no confusion on what was brought in/paid out).
You'll most likely be in charge of promo if it's "your" show, but it's totally acceptable to ask the openers to pull their weight in promo as well (ESPECIALLY if they are getting a cut of the door). However, if you're paying them a flat amount, you can't expect they go all out with promo (because they have no financial interest in this show's success). So, it may be to your benefit to give them a cut of the door.
Now, if you're headlining on tour with local openers it's a little different game. The promoter or venue who booked the local opener will most likely negotiate the deal with them. Confirm with the talent buyer (promoter or booker at venue) the opener/s set times/length and make sure your pay is not affected by theirs (or if it is by how much - some talent buyers take their pay off the top of yours). I always like to get the local opener's contact info and touch base with them a couple weeks in advance and get them excited for the show and help promote it. Remember, you'll most likely need some of the local opener's audience to fill the venue out.
When you're at the club make sure to go introduce yourself to the local openers (sometimes they'll be huge fans and too nervous to come say hi). The short amount of time you have with them will leave a HUGE, lasting impression on what kind of person you are. They will either turn into lifelong fans or vocal haters. Hang out with them in the green room. Get their story. Take a photo with them and create a "new friends" Facebook album on your Page. Ask them to tag themselves in it (do it in the green room) and I guarantee they'll get all their friends to check out this photo on YOUR Musician Page (more Likes for you!).
And WATCH THEIR SET! If you're reading this I'm assuming you're not at rock star status where if you show your face in the club the entire place will lose their shit. It's totally reasonable to hang out on the side of the stage and not in the house as most people in the house will want to talk with you (which takes them away from the opener's set - and you should preserve your voice if you're the singer), but watch at least a couple songs and compliment the local opener on their music. It means a lot more than you may think.
It's also a good idea to have your tour manager (or you if you don't have one) to go over the set times again (just to make sure everyone's on the same page and make sure they won't go over their time...again... they may not have read The Opener). I've had headliners in the past (when I've been the local opener) ask me before the show to not hang out by the merch table when the headliner is performing. If it's a small club, this will be very noticeable (and quite annoying) to the headliner if the opening band is taking photos and chatting with fans during your set (especially if it's a quiet singer/songwriter show). It's fine during set breaks, but once the show starts the opener should go hide at least for a few songs. Be polite about this and feel it out. If it's a big club, this is probably not necessary to mention and the openers could take this the wrong way.
Every band believes that once they become a headliner they will always remain one. This isn't the case no matter who you are. So, make sure you treat your openers with respect - they may be your headliner someday very soon.