Most of the things I know about mixtapes I learned from High Fidelity. And honestly, the rules are pretty simple: start strong, bring it up, chill it out, and follow through.
Still, there's no harm in aspiring to perfection, so I thought I'd explain what my favorite way to form a 12 song playlist is. It's not perfect, and at times I'm making it up on the spot, but that's kind of the case with most things I do.
1.) Start Strong, Start Weird: Unfortunately, there is no such thing as a universally strong song. For instance, the strongest song to start a mixtape to me is London Calling by The Clash because it's weird, starts way too loud, and is still a recognizably popular rhythm. To you, it may be What Do You Mean. The point is, the song has to be unexpected and pleasant, like ice cream for breakfast.
2.) Push Harder: I agree with High Fidelity again, you have to bring it up from here. Please note, faster does not mean the song is more intense. You could have the most jumbled guitar-driven anthem here if you wanted but the it wouldn't mean anything unless the song builds off the previous song to a rising crescendo of melodic energy.
3.) Don't Be An Asshole, Asshole: Here is where logic dictates that you take a light downhill to let the person you dedicated this album to catch their breath. On the one hand, you could make the choice to include something soothing and chilled out that doesn't mean much, and you could probably get away with it. But that's not what this album is about, it's about you telling someone how much they mean to you! Take the time to find out their favorite acoustic record, look through your texts to find their favorite song off that record, and let their body relax while their heart soars.
4.) Life Is Made Of Wonder: Having just done something nice for them, do something nice for the album. Play something so bizarre, so out of touch with everything in the taste of the album, that they know it signals they are over the first wave. This song is a narcissistic endeavor, a song to help you meditate and reengage in your music selection.
5.) Love Is Intimate: If this album isn't made for someone you love, then why are you making it? This is the song that represents you both, and becomes a transition away from the album being about just impressing them. The playlist evolves from a boombox under the window to a soft confession on the balcony.
6.) Vulnerability Is For Squares...: You just bared your heart, play something to make them smile. It shouldn't be over three minutes.
7.) ...So Make Them The Square: This one is tough, because you have to guess exactly right what song meant so much to them that they don't even talk about it. I'd recommend looking back at their Facebook posts, or taking a shot in the dark based on something they mentioned offhand a while ago. Low success rate, but it's always worth a gamble.
8.) Damage Control, One Way Or Another: Following that previous song, there are two possible outcomes. They are either feeling so happy you could play Nickelback and get away with it, or they are pleasantly nonplussed by a band they happen to like quite a bit. Either way, this one's a good time to play it safe, back off a bit, and pick a nice, neutral song.
9.) Narcissus And His Damned Swan Song: This song is your final one that is just about you, and at this point should introduce a part of your heart they didn't know about. Whether that be your unresolved taste for Punk or your love of Beethoven, it should be something deeply important to you that they don't expect. Hell, it may even be something you don't expect.
10.) Yes, I Can Hear You. Can You Hear Me?: They deserve a nice, mellow song by a band they like. I recommend something in the Alt J range of interesting and chill, but that's just me.
11.) This Is Our Song, You Know: At this point, you're reaching the end of the album and need to put in the song you both love. You know, the song that came on the first time you danced or some sentimental thing. Maybe it's just a song you think you would both like, but that's up to your discretion.
12.) How The Hell Do You Spell "Epitaph?": The most important songs on an album are the opening and the closing. This song should be like the heel on a loaf of bread: tough because you are realizing it's over, yet crunchy in its own way. Have a nice, long reverb on the final chord and close the doors.
13.) But Wait, There's More!: Except the thing is, no mix is complete without a twist at the end. The final, bonus song on your album should be something that challenges the recipient to match and better the mix. It's a confrontation of sorts, but also an affirmation that you two are whatever you are, and that will not change. Invite them to return your correspondence, and see what happens.