Let's add some more basic things a dj needs to know about mixing:
First, good dj beatmixes only records that are in same
key. If next song is going to be in
different key, don't mix it. Wait until appropriate part in
song structure to
fade previous song out and start new one after you've faded the first one out.
If dj plays songs that have lyrics, then it's appropriate to play even couples of songs that have similar lyrics. Creating a continuous story with song lyrics in the mix is a skill that only few can do.
Never let two vocals to play over each other; that sounds awful.
There is this unwritten law among dj's that songs shouldn't be pitched over +4 %, or -2 %. This way songs will sound as they were supposed to sound like when they were created.
Know your tracks. Most dance music tracks have specific parts that are meant to help the dj to mix in a new track. This kind of part can be identified usually when the instruments in song are reduced to rhythmic parts only, so there's no main melody and possible lyrics at all.