Hone your talent. No one wants to sponsor or endorse a mediocre band. Practice religiously and make your music the best that it possibly can be. If you feel that you aren't improving, consider taking music lessons.
Build your fan base. Play in public to reach a wide audience. The bigger the following you have, the more likely it is that you can make money for companies who might want to invest in you.
Visit small music businesses and luthiers (someone who makes stringed musical instruments, such as a guitar) in your area. If you get to know these people personally and they like your band, they may consider lending a helping hand.
Play the largest venues that you can. Potential sponsors are looking for the greatest publicity possible.
Check entertainment companies (such as alcohol brands and instrument businesses) online to find out their criteria for sponsorship. Many companies sponsor a relatively small number of artists each year in exchange for those artists advertising for them. If a company's website doesn't give information on sponsorship, email or call them to find out more details.
Talk to local businesses. Offer to place their name on concert programs,T-shirts, guitar picks or CDs in exchange for some their sponsorship or endorsement. Supply them with a recording of your music if they haven't heard you before so that they can decide if they like what you do.