Choose a few concrete words that describe your genre. Some bands don't like being "pigeonholed" to labels, but it's important that you choose some descriptions for people to refer to. Think about what bands you sound similar to, as well, which not only helps you decide what type of singer you're looking for, but will also help your potential future vocalist decide whether your band is even appropriate for his or her style.
Set up a date where your bandmates and yourself have the day off, so you can hold auditions. Pick a place and a time and then make some flyers. Post the flyers in areas such as your local music store or near your local college or community college music classes. Make sure you include what type of band you have on the flyer, what kind of vocalist you're looking for, what the band has accomplished in its career, and where you plan on going.
Post updates on your social networking sites (such as Facebook, Twitter, and MySpace) that let people know you're looking for a vocalist. The more people who know you're looking for a vocalist, the better chances you have of finding someone. Give vocalists a way to contact you if they're interested. Also, have your music or a description of your music ready for candidates if they want a clear idea of your sound.
Ask your friends if they know anyone who sings well. Tell your friends to tell their friends that you're in a band and looking for a vocalist. Word of mouth is sometimes the best way to advertise because it spreads information fast and is free.
Hold auditions and jam with the different vocalists who want to try out. Have one of your bandmates or friends take notes while you rehearse so you can refer to them later. Note each person's vocal style, image, talent, and chemistry. If you narrow it down to a few choices and still can't decide, then call the finalists back and have them do one more audition.