Most of my a cappella quartet gigs are booked 3-10 months ahead, which suits us well. We intend it to be a part time thing and so far--knock on wood--we have no trouble meeting our income targets. Twice a year (roughly January for spring-thru-fall dates, and May or June for fall-thru-spring dates) one of us e-mails our list of previous gigs and new prospects; this always includes a 60-second sampler mp3 that blends clips from 6 or 7 songs characteristic of our full repertoire. Only a couple of regulars hire us by return e-mail while the majority of bookings result from follow-up phone calls. Where applicable (i.e. for organizations) we do check first to obtain the name and direct e-mail address of the person who books talent. I'm starting my 3rd year with this group and we have a blast!