Know what music is hot right now. Monitor the Billboard music charts and the current most-downloaded MP3s on Amazon.com or iTunes. Knowing what music people are currently listening to and demanding will give you a good idea of the songs or artists that people will most likely want to hear.
Work on improving your mixing skills. Practice mixing tracks often with sound editing and mixing programs on your computer. You can also go old school and practice mixing and scratching records on a turntable. Experiment with different songs and try mashing two together into one new track.
Prepare multiple playlists before you work a gig. DJing can be a hectic job that is quite distracting at times, especially if you get caught up in socializing with people. Have some of your work done ahead of time by having a few pre-made playlists on hand that you can use as a starting point. You can then insert requests and other music into the playlist as the night goes on.
Play a diverse mix of music. Play songs that are familiar to people as well as lesser known, more obscure tracks. Alternate between high-energy songs and songs that are more mellow so as to give the crowd a break from dancing. Play a mix of older songs that everyone knows and new tracks that are currently hot on the music charts.
Go with the flow of the crowd. Monitor the crowd to see how they are responding to the music you are playing. Note if a lot of people are dancing and having a good time, or starting to leave. Identify what genre or artist the crowd seems to be most into, and steer the music you play in that direction. While you will most likely go into a gig with a certain playlist in mind, it's best to remain flexible.
Socialize with the crowd. Come out of the DJ booth from time to time to get a drink and to mingle with the crowds. Make small talk with anyone who comes up to you. Take requests from people, which almost always pleases the crowd. Meeting people is also a great way to get your name out there and book future gigs.