Most of the Djs do this by instinct, or by "it sounds good like this".
But some of us are serious about keys, and more and more as new djs are digitally oriented and can pitch shift/ pitch lock easily.
For remembering the keys, the dj softwares have now a field to note it down. And an old animal like me sticks notes on the vinyl cover itself (bpm, key, and the track/side that I like the most).
http://soundcloud.com/bat-manson for my sound if you drop one comment I would be pleased.
I agree with suzzymackenzie, at some point, you'll need to be a caricature of your style and be unique. Help people to remember your tracks easily. For instance, the sample you put in the middle is a good idea. At the end, that's mainly a melody story I would say ...
You have a pretty good level anyway (or I wouldn't bother answering you), I just try to give you new ideas to come up with and be better imho, I'm not bitching around
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