There are a lot of good/great DJ apps for iOS, but if you’re ready to graduate to physical control over your mixes, the iRig Mix ($100) is a good place to start. This lightweight, portable box offers crossfader, cue, EQ, and volume controls, can be used with a single iOS device or in a more traditional two device setup, and also offers an extra guitar/mic input, standard RCA outputs for direct connection to PAs or powered speakers, and, of course, a headphone output for master or cue monitoring