Try to use tracks that fit together and maintain the flow of the mix. I like mixes that slowly but surely increase in intensity... or the other way around, you could start energetic and gradually let it become more chilled..... or whatever you want
A lot of tracks in the chill/ambient genre may have short or longer (beatless) intro's and/or outro's, that are suitable for mixing with the next track with a simple crossfade.
Like this: when track A has 'noises' (without tones/melody) at the end (or the point where you make it end), you can often easily mix that with track B, that also has 'noises' OR tones/melody at the start (or the point where you make it start).
When both tracks have rhythms and melodies all the way, it's gonna be (very) hard to mix chillout music and still make the transition sound smooth and harmonic.
IMO the coolest thing that can be achieved with mixing this type of music is when both track A and B have tonal/melodic qualities at the end/start and form a sort of harmony together during the transition.
I hope that made any sense lol