Most playback software comes with cross-fading support, which can be turned on. When enabled, this takes place automatically. While a DJ based program requires a DJ to do the cross-fading, the purpose here is that the DJ can align the beat of the current song to that of the next song by listening in its headphones before doing the cross-fading to the PA system. An automatic crossfader doesn't align the beats.
Here's how to turn on the automatic cross-fader in Windows Media player, Foobar and Winamp:
Windows Media Player
- Right-click on the titlebar, select View -> Enhancements -> Crossfading and Auto Volume Levelling
- Click "Turn on Crossfading
- Drag the slider to set how many seconds to overlap.
- Go into the File menu and select "Preferences"
- Go into Playback -> DSP Manager
- Select "Crossfader" and click "<="
- Click the "Configure Selected" button
- Drag the slider to set how many seconds to overlap
- Click 'OK' and 'Close'.
- Go into the Options menu and select "Preferences"
- On the left, go into Plug-ins -> Output
- Select "Nullsoft Directsound Output" from the top and click the "Configure" button
- Go to the "Other" tab and tick the option "Remove silence at the beginning.
- Go to the "Buffering" tab and set the buffer length to around 5000ms.
It's worth experimenting with a few songs to make sure you get the cross-fading to how you like it.
One thing to becareful with is songs of different volume levels. Most modern music is all of the same volume label, but if you're planning on using music from the 60's to the early 80's you're going to have a problem with some songs louder or quiter than others.
One option would be to experiment with auto-volume levelling in the software (e.g. Windows Media Player's "Auto Volume Levelling" on the same screen as the cross-fading) or checking if your PA system has this option. Another option would be to listen to the songs and try to set them in an order such that songs of a similar volume are grouped together in the playlist.