Beat matching becomes natural after a while, its like riding a bicycle or driving a car - you hardly think about it. A lot of DJs do play a pre-set mix. They pre arrange the tracks they will play but make changes as they go along.
I used to DJ myself and when I was playing on a residency I hardly prepared, I just had CDs with the latest tracks burned and carried around all my past CDs along. Then i'd continue where the previous DJ left off and depending on crowd reaction plan the next couple of tracks. Gave me a chance to experiment, and see crowd reaction to the newer tracks - if something put the crowd off, i'd discard it.
In a club it's sometimes a good idea to give a chance to people to visit the bar etc (some DJs from eastern european countries I talked to said the club owners demand this!)
As you get to know your music you know what 2-3 tracks match nicely together - this is what you gain from experience but new music is constantly released so you need to evolve with it. At the time when I quit, more tracks were being released every day than you could listen in a day - it became a full time job picking tracks to buy.
On the other hand when I got invited to play to a larger crowd usually outdoors, I took more care of what I was going to play and had in mind in advance how the set would work.