Flow, that nebulous concept that turns a bunch of songs by different people into a cohesive mixtape, really is vital to the success of a tape (or even, if you're a professional producer, an album). When each song seems like a natural fit, the listener won't stop to question their presence on the tape. Even better, the listener won't have to work to figure out what's going on, thus allowing more time for the important things: enjoying the mix and discovering new music. But achieving flow is no mean feat, even for the experienced mixer. Luckily, there are a few ways to fake flow or even avoid it altogether.
1. Use songs that start with a percussion-only intro.For whatever reason, drum solos seem to have the same musical effect as a palate cleanser has on taste. There does have to be
some measure of similarity between songs - it likely won't work, for example, to go from a down-tempo singer-songwriter kind of thing to a hardcore death-metal drum onslaught - but in general it's easier to bridge a gap between tracks with percussion than with notes.
2. Use an instrumental track.Again, the reason why this works is not entirely clear, but songs without lyrics seem to work well enough as curveballs that they can be used in a pinch as transition tracks. The same rule applies, though: don't go too crazy.
3. Use a spoken-word track with no music at all.Whereas the first two are merely tricks, this is pretty much out-and-out cheating. Even better, the words themselves don't seem to matter at all, so you can really let your imagination run wild with this one. These can be skits (like rap albums tend to feature), poems, quotes from movies or TV shows, lines from speeches - basically anything works. It seems like these would have to relate in some way to the preceding or forthcoming track, but I promise you that isn't the case. Listeners will be able to tell, of course, when they do, so maybe there are some bonus points to be had, but a connection really not necessary.
4. Call one group of songs "Side A" and the other "Side B."Despite the fact that this distinction has been obsolete for at least 10-15 years, enough people still understand it that it won't cause mass confusion. Even for those who don't, this should still work - the idea is to create a divide in the reader's mind so that the (hopefully relative) lack of flow between two tracks isn't a distraction. Ideally, this would only happen when you actually planned for it to, but sometimes there isn't another obvious way out. The only caveat here pertains to side length, because if Side A has 8 tracks and Side B only has 2, it's going to look pretty suspicious. It doesn't have to be 50-50, but the relative lengths shouldn't be too far off.
There are almost certainly other ways to address flow problems, but these are the four best that I've found and they should work in most situations. If not, though, there's no rush - it's always okay to start a mix over from scratch or to put one on indefinite hold until inspiration strikes. Or,
to quote Darren Aronofksy, "You remember Archimedes of Syracuse, eh? The king asks Archimedes to determine if a present he's received is actually solid gold. Unsolved problem at the time. It tortures the great Greek mathematician for weeks - insomnia haunts him and he twists and turns in his bed for nights on end. Finally, his equally exhausted wife - she's forced to share a bed with this genius - convinces him to take a bath to relax. While he's entering the tub, Archimedes notices the bath water rise. Displacement, a way to determine volume, and that's a way to determine density - weight over volume. And thus, Archimedes solves the problem. He screams 'Eureka' and he is so overwhelmed he runs dripping naked through the streets to the king's palace to report his discovery."
