Listen to the structure of the music that you have picked out for your mix. Listen to the rhythms, the BPM (beats-per-minute) and the melodies, and listen for any breakdowns or pauses that would make a good spot to begin the actual blending. Once you've gotten the feel of the tracks, start counting the bars (every 4 beats) and the phrases (every 4 bars) in order to get an idea of how you want to bring in one track and take out the other.
Write down the BPM of the songs in your mix on their respective record sleeves or CD jackets. That makes it easier for future reference.
Keep your hand on the pitch control as it is a key factor in getting two tracks to synch up. Listen to the cued song (the song that's going to be bought in) through one of the channels of your headphones and get it into place. At the same time, use your free ear to keep track of the live song (the song being played to the crowd). While you're "listening" to both tracks, use the pitch control on the cued song to slow it down or speed it up in order to match the beat or the rhythm of the live song.
Keep in mind that just because you have let the cued track go and it's (initially) in sync with the live track, you still need to keep your hand on the pitch control. The two songs you are playing are similar, but not the same. It will not take long for one track to take off and desync with the other, creating a messy sound and an embarrassed DJ.
Have fun with the crossfade. DJing is all about experimenting and you never know what you'll come up with when you're just randomly going pack and forth between two tracks.
Experiment when blending two tracks together. Play with the bass, the treble and the equalizer on the mixer when you're in the process of bringing in the cued track in order to make a livelier segue. If you're playing the same genre of music all night, it'll get a bit boring just going straight from one track to another. Messing with mixer creates what sounds like a third song, and gives the mix (and the DJ) a bit more personality.
Listen to the uniformity of the beats when mixing genres like house, techno, EBM and/or trance. All those genres utilize a steady four-on-the-floor beat that can be easily synced up.
Listen to the high-hat and the snares when mixing hip-hop, soul and/or drum and bass.With these genres, the bass and the beats are all over the place but the higher drum sounds are usually in a steady pattern. The high snare sounds will act as the signal as to when to bring your next track in.
Be careful when you are mixing two completely different genres with completely different BPMs. It may sound silly, but it is very much a possibility to make a smooth transition between styles like funk to house or even country to electro. However, the two tracks need to have a similar feeling, pace and/or rhythm in order to make a good segue. You don't want to go from a bright, happy tune directly to a dark, brooding tune. Listen for a good breakdown or pause in the live track as a signal to throw in the cued track.