Name your files consistently
Organisation
A good pattern is '01 - Track name.mp3', so they sort by themselves
Keep artwork small
Performance
Images of 500×500 px at most keep everything responsive
Tag your files
Tags
A tool such as Mp3tag fills in the ID3 tags properly
Build playlists around a theme
Playlists
Sort them by mood, genre or occasion to find them quickly
The ideal setup
Two screens
Touch interface on screen one, videos on screen two — a proper DJ setup