When you make a music mix, and you really care about the sequence flowing well, there are two skills that help tremendously. They both depend on recognizing that a song you like is not at its best here (regardless of how you feel about it, how much you admire it, how well you might think it works in theory); you know it's dragging the gestalt down, putting a snare in the fabric. So when you see that snare, you either 1) let go of the song entirely, because it isn't suiting the mix, or about equivalently you toss it on the backburner for another time, or 2) you find a great transition song that helps lead the previous song into this one, so that this one shines at its best.
I'm better at the 2nd. It's easier for me to find good transitions than it is for me to let go of things entirely.
Sometimes this means that song A not quite leading well into song B becomes like 10 or 20 songs in between A and B, as I try to get B to sound really good after something.
As for the core skill of recognizing whether a song actually fits, I break it down...
First, I play the previous song in its entirety. Ideally, actually, I play the 2 or 3 previous songs in their entirety, back to back, with no interruptions. And I pay close attention and feel as much as possible. (Um. Cannabis can really help. Just saying. And using a service with advertising can really get in the way. Paying for Spotify makes it 50x easier for me to come up with good sequences. It doesn't seem intuitive that a small thing would help that much, but for me it does.) At this stage, I am usually hyperfocusing on the music above all else. (Lyrics and video are distant second and third concerns. Music is music, and I'm very music-centric. Hey, I love music. I don't listen to music for lyrics or pictures. Those can be nice, even great, but lyrics aren't music and pictures aren't music.)
Second, I listen to the song in question carefully and assess whether I actually WANT to listen to all of it, and beyond just THAT, whether I think it sounds at its BEST. If I want to skip the song right now, it's in the wrong place. (Going back and forth comparing positions, hearing the same songs over and over and over and over, adds a real layer of challenge. Am I sick of this song because I've started playing it 20 times today, and listened all the way through maybe 7 times, or do I want to skip to the next track because this one is not in the best position to shine? I have to err on the side of assuming the latter. That's part of the secret. If I've heard a song 20 times today and NOW, in THIS position, it sounds awesome and I want to listen all the way through again, I've hit the nail on the head. Bingazzimo!)
Third, I assess beginnings and endings. How do the first few seconds land after the previous song? If it's jarring, that might be ok (strong contrasts sometimes work), but it might also suck. Anything jarring is important to pay close attention to. If the song sounds great in its entirety and the lead-in from the previous song is just *chef's kiss*, then I will be very reluctant to let go of this pairing. Two songs that, back to back, sound like one piece of glorious music - that's what this is all about - that's what I'm looking for all the time, so when I find it, I boost it.
Fourth, I pay attention to the story arc of the sequence. What impression do the lyrics give? What tale is being told? How are the lyrics in this song affected by the lyrics in the songs before it? How will it affect the lyrics in the songs after it? Most of the weighing I've described in previous paragraphs is purely musical (though lyrics will work their way in at the edges of consciousness). At this stage, I put the music aside for a moment and pay full attention to the story. Am I embarrassed to let this story arc see the light of day? And if I'm embarrassed, is that ok? Is it expressive and interesting? These are my main concerns now.
Fifth, I make sure that the audio availability and quality and the visuals and the video's theme are all working for me. I'll find the link with the best quality of sound and image, and if applicable, the video I enjoy the most and find the most fitting here in the sequence. Sometimes a great song has to be dropped because there's no YouTube link, or the quality is just not up to par, or (very occasionally) the video is just too off-putting. For the latter, sometimes it's enough to move the track to later in the sequence (given that I'll usually have finished and shared most of the sequence up to this point already, later is still an option, but earlier isn't).
That's basically my process. I keep at it until I'm really happy with a stretch of songs... or, sometimes, until I'm happy enough and kind of fed up and just ready to move on.
I try to keep in mind the idea that every song should be something I'm overjoyed to share... the kind of thing you keep playing over and over, and you're just desperate to find someone else who enjoys or would enjoy it, because you want to share your sonic passion. So my ideal is that every song I include is that level of "amazingly, irrepressibly good." An earworm. An anthem. An era.
In practice, I let too many less-giddying songs into the mix. Or I let songs into the mix that I have once upon a time felt so intensely about, but not really recently; I know that I can feel that way about them, so I assume others can; I figure if I get the song to fit nicely, it could be someone's favorite song of the week, even though it isn't mine anymore. Those compromises may not be good, or maybe they're ok or good but not great. So I try to avoid them, but I will accept some of them to keep things moving, to get great songs to transition into each other. So there's an aspect of using songs I'm less currently passionate about as filler, but I'm aware of that possibility and try not to include any filler at all.
When I'm having trouble finding the next song in the sequence - nothing seems to fit after it that well - often what it takes is just plain discovering a new song I'm in love with. The next time I hear something I'm desperate to share, I find there's a good chance it will fit perfectly, or at least very well, after the problem song I've been trying to figure out how to follow up. It's almost as if the problem of "What song goes next?" takes up residence in my head, and I go about my life for a few days or weeks until something captivates me. Even if I wasn't thinking about the problem or that song, I think sometimes I'm really captivated by a song because it's sort of the next evolution in the story I'm telling.
I don't know how other people make mixes. But that's how I do.
Oh, maybe there's one other thing to mention. I love to mix genres. That's kind of built into my musical assessment. If I can put a piece of classical music (obscure or famous, doesn't matter) next to, say, a repetitive piece of electronic game music (obscure or famous, doesn't matter) and get it to work fantastically, then I'm overjoyed. The bigger the contrasts or leaps, the better, often. But what takes priority is that the mix actually works. So often I will follow up a song with a similar-enough song to continue some sort of mood and groove. But that is only - strictly only - when I want that mood or groove to continue. As soon as I'm the least bit tired of the mood or the groove, I'll change it - and as drastically as I feel like. I feel no obligation to suit an audience's expectations, and, actually, given the choice, I'd prefer to upend them delightfully.
