I think its just something that comes over time, trust your ears, if they think it sounds right chances are it is right. Start to think about the mood of a track – is it dark / light / hard / soft / percussive / synthy how does it make you feel? go through your collection and find something that makes you feel the same way then load them into your decks and experiment. A good technique which i use is by thinking of tracks in sets of 3. Find 3 tracks that go really well together and have similar elements (look at mood,genre,key,bpm,energy) then another 3 and another 3 etc etc then you will have a bunch of “mini sets” you can turn to at any time, you might end up with a set of banging tracks and a set not so banging but somewhere along the line you will find a track that can help bridge that gap – that slots in the middle and suddenly it all comes together. Im not saying stick to this rigidly of course but doing this process will get you to know your music collection and evetually it will come naturally to you
