I have a hard disk full of under-2 minute song which I now want to put all together and make something useful out of it. The hardest part is to make them sound like they naturally belong together by composing transitions between them.

Now I need the help and advice of all the learned ones hanging out here to help me out. I already can think of a number of ways to do the transitions. By my lack of formal education I just give them my own names. There is the "cross-fade" where some essential part of the next motif overlaps with the previous one. Then there is the "cut" where the transition is sudden without much extra composition required. These can have variations like "fade to silence - fade from silence" (pretty obvious this one), etc. Harmonic transitions (e.g. going from a key of C to E) also have a few variants like circle of 5ths, pivot notes, extended chords (7ths, 9ths) etc.

With my few variants I can think of and the number of cues I'm adding together it soon starts to sound the same.

Is there some classical rules about transitions? Are there any opinions on the different ways to do it?