As an aspiring musician who's hoping to finally drop that album in a few months (or sooner, since my mother has requested I record all of my songs for her for Christmas anyway), you'd think that I'd be able to answer this question fairly easily, or at least get a good answer out after a bit of thought. Unfortunately I'm drawing a partial blank here. The structure of music has never been something I've really understood. Nah, it's more of something that I feel, so to speak. When I write music I rely more on instinct than theory. That's helped me with improv (improvisation) greatly, but it's slowed me down when it comes to getting the song down on paper or writing a fully orchestrated piece.
Regardless, I'll try to scrounge up some advice/thoughts real quick.
Writing music is, in a way, a lot like writing stories. I know that may sound strange (or not), but trust me on this one. When a song starts out it has to have a sort of hook. Earlier you mentioned the Imperial March. As you probably remember, the intro starts out almost as memorable as the main portion. It's a good draw-in because it starts off strong. The fast nature of it grabs your attention, the symbol keeps it, then the melody comes popping in, grabs you by the throat (with the force), and leaves you wanting to nod along with the music. A slower example of this is the main theme of Fallout 4.
The song starts off slow. A single piano note (a high one) gets your attention. Then a chord brings the volume up just a tiny bit. As it continues the piano plays a sort of somber tone, while the violin adds a sense of hope (using musical theory stuff I don't remember). There's not much of a transition as a drum beat pulls you in farther, accompanied by a lovely set of chords (du do doon) that gives you that sense of "the world has ended". That pattern repeats several times throughout the song. Little bits are added, the drum picks up, the violins come back, and things mix together very well. For this song, the sense of emotion created by the clever use of major chords throughout a (as far as my ears can tell) mainly minor song (though it gets more into the major stuff towards the end).
That brings me to my second point: Emotions make a song memorable. Jurassic Park's theme fills you with a sort of wonder. It starts off slow, then rises up, continuously using major chords, and brings in yet another short pattern to repeat throughout. Fury Road's theme combines a heavy drumbeat with fast-paced strings (and techno stuff) to bring a sense of urgency, a sort of rush, to the song. Time, from Inception, does a wonderful job of seeming nostalgic, thoughtful, and, to me, sad. In addition, it builds up, really bringing things to a sort of musical climax. However, it doesn't overdo it, but keeps the volume (and speed) at an acceptable level (compared to the rest of the song).
Thirdly, that brings me to the idea of the 'structure' of a good song. Much like a good novel, a song will have multiple parts, such as: the intro/hook, the rising action, the climax, the falling action, and the resolution. Some songs have a very big climax/a point where things get quite intense (think about bass drops in dubstep), while others have a shorter, sweet high point. Sometimes we'll remember the high point of a song, other times it'll be the peaceful rising or falling action that will stay with us. A lot of musicians will have a short pattern that they'll repeat throughout a song (we can compare those to main characters if we want). Often times a climax will include that pattern, but stronger, with a few tweaks or additional parts to it.
The ending of a song, while not often memorable, can be important to leaving a listener satisfied (which can, of course, make the song as a whole more memorable). Depending on the feeling you're going for, there are two types of endings that I've seen go over best with crowds. One, my personal favorite, is the repetition of a single pattern as the song slows down, the volume lowers, and the whole thing fades out. A second way to end things is to end them with a single (or several) sharp, loud notes. Almost a sort of 'second climax'. This is usually better suited for faster, more energetic songs. It's like the last hurrah of a warrior, the last clang of one sword against another, or the final, best comeback/line in a rap battle.
To 'summarize', a few good points to a memorable song: The hook, the emotions, a good structure, and a good ending. Now, making sure your song checks all of those off the list, well... that's another thing, for another day (perhaps). For now I'll just leave you with this 'slightly' long post.
[member="Mark Sage"]
, but its how I remember songs for inspiration.