Play live shows on a regular basis. As well as helping you improve at your craft, it also gets your name out in public and exposes you to networking possibilities with other musicians. To maximize this, make an effort to interact with audience members before and after your show.
Make a quality demo recording of your best three or four songs. Telling people that you have quality material and are looking for background singers isn't likely to be too fruitful if you don't have recordings to back it up. Always carry CD copies with you so that you can pass them out to interested parties, making sure that your contact information is clearly marked on the CD.
Make a detailed list of the songs you play, complete with lyrics and chords in the keys you play them. Make plenty of copies so that you can hand them out to interested candidates before any audition or jam session. Helping singers come fully prepared is a great time-saver and will make you appear more professional.
Visit local music stores and ask if you can put a flyer or business card up on their notice board. In addition, make an effort to befriend the store employees and let them know that you're looking for background singers. Since they deal with musicians on a daily basis, they may be able to pass on your demo and contact information to likely candidates.
Place classified ads in your local music publications. Many of these, especially online classifieds such as Craigslist, are free, but to save yourself time, be clear and precise in the wording of your ads. Specify your genre of music and what your musical goals are, so that you don't waste time corresponding with incompatible musicians.
Participate in open-mic nights at local establishments. Since so many of those in attendance are there to play and sing, it's a great opportunity to mingle with fellow singers. As an added bonus, you're able to see them perform before an audience, so you can quickly discover which performers might be a good stylistic fit for your music. If someone impresses you, ask if they'd be interested in performing with you at an upcoming open-mic night. This is a good way to initiate a working relationship without the pressure of a true audition.
Go out and support local performers at their shows. Introduce yourself and let them know the kind of singers with whom you're looking to collaborate. If they're receptive, give them your demo CD and ask them to pass it on to anyone they know who's looking for singing work. Becoming friendly with working musicians will introduce you to their network of musical friends.