If you get a gig....

[luv2xra]

I make set playlist, which basically brings up the mood, in that playlist every track can be replace depending on the crowd. Basically, the playlist is just a frame that tells me how to mix on the fly.
Hopefully that makes sense hahaha.

[whiskers]

You should practice, practice, practice to not just hone your skill, but also know what songs go well with each other. So, technically, you can mix on the fly... but, you know kind of in your mind what direction and flow you can take from all that practice.

This is what DJing is about and I doubt that any program or time-coded vinyl or computer in the near-by future (except self-aware AI, of course) will replace the DJs ability to read the crowd, feel the mood of the set, know the tracks, and program the set.
Some of the most common advice is to never pre-program a set in your head. I tried doing that, it never worked so far, never more than 2 tracks (I'd play the two tracks that I wanted to play and then I'd go off on some crazy choices).

Also, don't get trapped in harmonic mixing. I have done some excellent non-harmonic transitions that worked well, and some crappy-ass harmonic transitions.