Creating a mixtape is all about creating a mood. The goal, if done correctly, is to take the listener on a journey, make them feel something, and leave them feeling better than when they started.
All of my mixtapes use the same formula which I will describe to you now. There are probably other ways you can accomplish this, but ever since I first came upon this formula I feel as though it has served me well. While not an expert on the subject, I have full confidence in the mixes I have made and given to people or posted online.
You start off with something to set the mood. Uptempo with a positive vibe, not the fastest or loudest track but something with a nice groove. This is what you play to while the listener enters the theater, finds their seat, gets settled in.
The second song is the most important song of the mix. You create a nice vibe with the first track and then you give it everything with the second song. Every now and then I will hear a song that, at that moment, feels like the most incredible thing in the world. The song comes together in such a perfect way that you want to listen to it again and again and never stop because it is perfection in three minutes and change. That is the song you pick for your second song. It clearly identifies who you are and what you are doing here.
From here your picks should remain super upbeat and fun. If you have fast or aggressive songs, this is where they go in the order. Assuming a 20 song mixtape, you want something like 8 songs in this portion.
Around the middle, track 10 of 20 or so, you start bringing it down. If you have a song with a climactic finish, where things just cut off abruptly this is perfect. The next portion of the song list is reserved for slower songs, ballads and acoustic numbers that would have been lost in the opening portion. I find this section is great for hip hop, as those tracks usually have a 100bpm tempo. Let the listener get another drink, sit down and relax a little.
As you approach the end of the tape, somewhere around track 15 of 20, you want to start bringing the mood back up. Songs with a rising action, or especially loud acoustic songs (bluegrass, loud folk numbers) are great for this. Your intention is not to jump right back into full-on crazy energy, not right away (and besides you put all your fastest songs in the first block right?).
The second to last song is key as it acts as a coda on everything that has come before. Think a band playing their "last song" when everyone knows they are going to come back out for an encore. Your goal in the third section, tracks 15 to 19 of 20, is to go from a quiet, relaxed mood back up to full energy by the time you get here.
The only guidance I can give for picking a last song is that you want to try to find something that will be especially memorable. It could be a fun song with a hook that you will be singing for days, it could be a sad song that you will always remember hearing. There are different criteria for picking track #2 and picking the last song of your mix, if anything those two should be the first two songs you pick before the rest of the track list comes together. If you do not have a great second song and a great closer, the rest of the things you pick do not matter.
There are a few other rules I try to follow that I think help provide direction. I always try to make my mixes no longer than 80 minutes (and shorter if possible) so that they can fit on a single CD. That said, 80 minutes is a long time to expect someone to listen, so think about who you are making the mix for and when they plan to listen to it.
I try to avoid reusing songs at all costs. It is okay to reuse a song or two if you are making separate mixes for different people, but picking the same #2 or last song on multiple mixes is completely verboten. The entire art of making a mix is to somehow make something completely new out of other people's art. When you start repeating yourself you lose any chance of impressing any magic into this thing.
The last rule is that I almost never ask people what they think after I give them a mix. This might seem selfish, but at the same time I feel it is selfish to assume the listener is going to spend nearly as much time analyzing the mix as you spend making it. They might, but more likely they might skip tracks they do not like or not pay much attention while listening at all. If they truly want to say something, they will.
Making a mixtape is like confessing your love to someone. Whether they take it or not, whether they listen or not, whether they love it or not, it is never your place to ask "Why?"