Distribute your demo CD or MP3 to local promoters and clubs. Focus on promotion companies that put on events featuring your style of music. For example, if you play deep tech-trance, a Top 40 promotion company is unlikely to be interested in hiring you as the warmup DJ for an event. Many clubs also hire resident DJs to play warmup sets so pass your demo out to the managers of your local bars and clubs.
Play tracks in a similar style to the main DJ's music. For example, if you're warming up for a house DJ, don't play drum 'n' bass. Keep the energy level of your set lower than you would for a main-event set. Focus on building a sense of anticipation rather than going all-out. Avoid playing popular songs that the main DJ is likely to include in her set. If the main DJ is also a producer, never play her own tracks in your warmup set.
Make it easy for the main DJ to take over. When you're nearing the end of your warmup set, bring the energy level down so that the main DJ can easily bring it back up. Finish your set with a track that's easy to segue into. For example, choose a closing song with a long outro made up entirely of percussion. Closing with this type of track makes it easy for the main DJ to begin with any track she chooses.