IMHO there is only one way to put medley's together and that is "right out of your head" and "on the spot."

If you have enough material in your head to work with, you read the audience and determine what "fits in the pocket" on the spot.

When I play, I'm always thinking 3-6 songs ahead depending on the mood of an audience at any given moment. Often (either by choice or by necessity) I'm won't know what I'm going to play next until I get to the last measure of the song I'm presently playing....that's how fast an audience's attention span will change. You really can't pre-program medley's as you have to be able to read and respond to a mood change among listeners.

Here are examples of how I'll put medley's together "on the spot."

Oktoberfest on Saturday. I'm playing the Blue Danube and the people are so into a waltz, I went right into Dr. Zhivago, and then Take Me Out To The Ball Game, finishing the medley with a short version of Ach Du Leiber Augustine.

Today at the Senior facility. I never know what I'm going to play until I have a chance to study the audience. I ended up looking out the window and seeing the sun shining, so I announced we were going to do "weather songs." Started with: Sunny Side of the Street, into Pennies from Heaven, then September in the Rain, slowed it down for Singin' in the Rain, Raindrops Keep Fallin' On My Head, and finally down to a crawl with an emotional version of Over the Rainbow.

Then to pick them up again, I did an "around the world" medley: Italian tarantella, Mexican Hat Dance, Scotland the Brave, Fere Jacques (France), Deep in the Heart of Texas, etc

Broadway (fast 4): I might start with Get Me To The Church On Time (My Fair Lady), then To Life or Fiddler on the Roof (Fiddler), end with Do-Re-Mi or I Could Have Danced All Night (both big fanfare tunes for going out with).

I find "impromptu" works best which is why I never use MIDI files. I need the freedom to follow an audience, rather than follow my MIDI file!