Don't worry about being terrified. You're playing bass, so you'll probably be inaudible!
Why do you only have one show in over a year? If you have stage fright, that's a good way to unnaturally intensify it. Get out and play more and I promise you that it won't be as hard to play, or to get a good sized audience.
As for packing the place, you're probably up a creek for this particular gig. For the next ones, here's what I've noticed that works for original bands. First, you should realize that there really aren't any "big breaks." Sure your friends will remember your band, but the vast majority of everyone else at the shows probably won't. I've seen hundreds of great bands that I can't remember the name of.
Second, spend some time reading up on marketing - there's a ton of websites and a few books that are specifically about getting your band out there. No one will go to your shows unless they know about you guys. Spend a year and play out like crazy, and always get people to sign up on an email list. While you're breaking down, get the singer or whomever is the most magnetic in the band to get people to get people to sign up on your list. Always have a merch table. One of the best things you can do to put your band's name out there is to have a really good looking T-shirt, especially one that girls will like to wear. I remember after going to one show to see a band I liked, my gf at the time bought a shirt she liked from the opening band called the Parson Redheads only because the shirt looked nice. I can't remember the name off-hand of the band I went to see, but I remember the other band because she wore the shirt all the time. Also, her ample bosom was a great advertisement for that band. Also, freebie stickers are an excellent way to get your name out there. I still go out and see a local band called Indian Fur Traders just because they had a cool looking sticker that I tossed on my bass case that reminds me how good of a band that they are.
After you can sense that people know your band, then only play in the same market once in a long while, but still play out in other nearby areas. All the while, be very active on facebook, and maybe Twitter if you have time, too. In general, try to market to girls and get them to your shows as much as possible. If you can get a bunch of girls to your show, the guys will always follow.
For this gig, there's a few things you can do, but there's no magic bullet (especially with the gig only about a week or so away). Your best bet is to really make sure all your friends are going. Other ways of getting people to a show when they don't already know your band rarely yield much in the way of results. They always say that the best way to build an audience is to already have one. It's kind of like "money makes more money." In any event, there has to be a local paper or website or something that lists local bars or venues and who's playing. For instance, where I live, it's an indie newspaper called the VC Reporter, as well as a website called venturarocks.com. Whenever I have a show at a bar, we make sure we're listed in the paper and on the website. Next, put up some posts up on craigslist if craigslist.org is often used in your city. Also, put up posters! You should start on this TODAY. Slap something/anything together and get to a copy center! A lousy looking poster is still better then no poster at all. Put them up this weekend at any shops that'll let you that might cater to the same people that might go see you play. Coffee shops, local record stores, local eateries that might be frequented by people who'd see your band, and anywhere else that you can think of or will let you. Also, put out a few at the venue that you'll be at next week, and maybe even put fliers on car windshields that are at the place tonight.
Also take note of bands that are where you want to be, and copy them, such as how often they send out band-related emails and what they say, or what sort of twitter/facebook posts they put up. Let people see that you're real people - post pics of a recent trip, or if you guys got burgers on night, or whatever. Set goals that include timelines and specific tasks that'll get you to your goals, or I promise you that you'll not be where you want to be.
That stuff above will help you. However, if you only play one show a year or so, there's almost no way to really build up a decent sized audience. Get out and play more! You can still do things like open mics, jam nights, or even busking. All of that can get your name out more.
For a band to really get an audience, you MUST have a good stage show, a method of letting people know when your playing or how to buy your merchandise, and a method of reminding people why they should like your band.
Good luck and let us know how it goes!
Now, we're playing an actual show. For money, even. And this could lead to other shows -- I've arranged for at least two people to come and "audition" us that night to see if they can line up more shows for us.
