I like the transitions in your tracks Slignig. They're varied and involve a lot of different techniques.
Zooming out here for a bit and getting more general - so if I was thinking about things to contrast, I would think about (with a simple example to illustrate):
- Volume (soft to loud herp derp)
- Speed of rhythm, groove (fast notes to long block chords)
- Timbre (mellow plucks to aggressive growls)
- Recording fidelity (lo-fi to big clear sounds)
- Mix Space (Wide revberbs and panned instruments to narrow central sounds)
- Pitch, octave space (a melody that does not jump more than 3 tones, to a melody that spans two octaves)
- Voicing (one voice to a choir)
- Feel or vibe (delicate and melancholy to angry and assertive)
- Noisy to clear (tape hisses and vinyl samples, to sine waves and synthesized kicks)
Usually just by the fact that two separate parts are different they will have some of these characteristics, but it can be helpful to think about why they are different, and if there is anything you can do to promote the contrast.
Also, as Asylum seeker mentioned, the listener needs a thread of continuity running through transitions, so having one or more of these characteristics remain constant will usually provide that for you.
I got a lot of my transition ideas from analyzing Far too loud, these guys have a great grip on hammering you with heaps of different change ups and fills (they were doing complextro style quick cutting transitions before the style really caught on):