Harder with CDs but not impossible, for me where I bring in and take out tracks is always to do with what's happening in the track. I'll start bringing in a track maybe 32 bars or more before a break or some change in the track, and have it fully brought it right on the break or whatever, so it takes over much smoother. Basically you have to know your tracks really well.
I generally know my music very well. Even when the new phrase of the song I'm bringing in begins I'm still unsure when or how to bring out the originally playing track. Ease it out gradually up until the next phrase is usually what I do but when that next phrase of the incoming song starts, I cant seem to smoothly transition the song out. I'll either leave it playing in the background for too long not knowing the best time to fade it out or else il try and completely fade it out as the new phrase on the incoming track plays but it doesn't sound well. Think im going to record a short mix today, regardless of what it sounds like and upload it for tips.