Many people seem to have trouble with transitions, and I've noticed that it often seems to have the same cause - they don't understand how. I LOVE transitions, and would like to do my best to help. Hopefully if you're having trouble with transitions, this will help you.
(This applies to overall composition, and I'll gladly get into it in another post, but for this I'll just post a brief summary of the concept.) If you analyze music, it might eventually become clear that almost all good music has a direction - a purpose - somewhere it's trying to go. Musical flow is very general, and it depicts the overall feeling of the piece - the movement and changes that the piece goes through. If this flow is disrupted, it's often jarring and breaks the person out of their musical immersion. If the flow remains unbroken* throughout the piece, however, the piece simply brings the listener along for a ride, and they 'experience' the music rather than simply hear it.
Transitions are an important part of the musical flow. Most pieces of music have multiple sections, which might end up being very different from each other depending on what the musician is trying to do. The most common place the flow is disrupted is during the transitions between these sections. If you notice, in a piece without good transitions you can tell precisely when one section ends and another begins - and it breaks you out of your immersion every single time until you're used to it. The flow should be retained between sections during the transitions - INCLUDING the intro and outro. The outro of a piece is more important, as it keeps people from having a rude awakening from the immersion. The intro is not as important, but is still relevant. Remember, the intro and outro are transitions too - from silence to music and from music to silence.
Now that the more artsy-fartsy thoughts about transitions out of the way, here are more mechanics. When transitioning to and from a section, there needs to be some kind of buildup or preparation for the transition. This is not just a reversed cymbal, but more buildup in the composition, often a rise in tension and possibly volume. Some ways to do this are using pitch slides, arpeggios, introducing some dissonance, and/or changing the chord progression slightly. More advanced transitions have foreshadowing, where there is a mock transition earlier in the piece. Now the actual transition comes. The actual transition is commonly some kind of momentary climax in the piece (unless it is a transition to a piece's climactic section, in which case it's simply beginning the climax), though it could be something more unorthodox. Reversed cymbals, cymbal rolls, and white noise sweeps are commonly used. Drum fills and arpeggios are another good way to do so. There are other ways to transition, but those are some of the simplest and clearest. Finally, the end of the transition. There should be a release to the tension as well. Some ways to do this are by having the drums use crash cymbals, deep thuds (kick with low end reverb), or other sound effects. For instruments only, you can complete the arpeggio or use fortissimo octaves for emphasis of the first note. Note that all examples used in this are not the only ways of achieving the effect specified - there are more ways than those stated.
This is something that many people forget about, possibly because it's a bit more difficult than transitioning between sections. There is no way to ride out the end of the transition, so what is needed here is some kind of conclusion. The piece almost always needs to feel like it is over, so people will feel the winding down**. A fadeout is often used, but it is a lazy way of completing a piece of music. Some ways of concluding are using a large chord, having an effect to 'wind down' the piece (such as pitch-sliding everything down or up), using filters to take out instruments by high or lowpassing them while fading them out, or simply fading out a single instrument after 'concluding' the piece.
Most people have reasonable introductions, and this is not as important as the transitions within the music or the outro, but it is still good to be able to do. The intro should introduce the piece, coming in from silence. Suddenly introducing instruments might or might not work depending on the instruments. Typically if this is done, there should be a filter of some kind introducing the instruments from a quieter volume. Drum fills make good introductions, as do pads or legato strings.
While I don't normally make Club Mixes, they typically have a specific kind of intro and outro - a four-on-the-floor bass drum. This is meant to allow DJs to transition easily between different songs. It's not overly difficult to pull off, as you are no longer transitioning from silence. Remember, the four-on-the-floor bass drum is just how they typically begin and end - it is NOT what they ALWAYS do. That said, I would recommend having the intro and outro be able to transition with a four-on-the-floor beat.
Hopefully this was of some help to you, even if just in general. If anyone wants specific examples of transitions in the context of musical pieces, I'll edit some into the post!
If this was helpful and you think someone else could benefit from it, please show it to them. :) If this gets a good response, I'll likely post it on the Music In Simple website when that is created.
Any questions or edits you think could be made? Please PM me.
*Pauses in the flow of the piece and breaks in the music CAN be used within the flow to good effect. However, it is difficult to do this and many people won't be able to at first. If it fits in the flow, go for it!
**Some conclusions can feel unfinished depending on the intentions of the piece. In club mixes this applies, so see Item 6. It can also apply in jazz, but that is in more advanced compositional theory and will not be covered here.