You really need to learn and understand Beats, Bars and Phrases. 4 beats to a bar, 8 bars (32 beats) to a phrase. When your mixing you want to start the track your about to mix in on the first beat of a new phrase. Most EDM tracks have a 30sec to 1min intro and outro. Find a few tracks that have the extended version and begin by layering the intro of the track you want to bring in over the top of the outro of the track playing.