Guilt your family, friends and coworkers. Your family and friends should be there to support your efforts. If you can't convince your mom or best friend to buy a gig ticket, you're going to be hard pressed convincing acquaintances, let alone total strangers. Use them to your advantage. Tell them it's a really big show, as any show is in the development of your career. Ask your friends to bring their friends.
Sell the show. If you're playing with a well-known band that people have heard of, play up this fact. This doesn't diminish the importance of your own efforts. It's using someone else's clout to further your own. This will be much more beneficial in your attempts to sell gig tickets to strangers who might never have heard of your band before. Highlight anything interesting about the show. If it's a Halloween show, emphasize the fact that not only is it a gig for your band, but a party on top of that.
Get out on the streets. Target your market. Go where people who might be into your kind of music shop, hang out and see shows, and walk right up to people and attempt to sell your gig tickets. For example, if you're in a heavy metal band, go to heavy metal venues and approach people there. Go to other places where heavy metal fans might hang out in your area, whether it be a tattoo parlor or an ice cream parlor. If your fans are in the youth demographic, hit the mall on the weekend. Don't be shy, or people won't take you seriously. Don't be afraid to really sell yourself and your band here. Work the people you are talking to. For example, if you approach a group of kids and one of them is wearing a Led Zeppelin t-shirt, mention how you like the band and how they might like your music if they like Led Zeppelin, even if you play in an avant-garde jazz trio.