Writing a prog rock song

No seriously, starting a side project and want to know how to write a prog rock song.

Clever.

All of the advice you've been given is solid.  I don't work around scales or modes or any of that.  I don't "strive" to play a certain way. 

I literally just mess around on a given instrument until I find a chord progression or melody or riff that is memorable and interesting to me.

If it passes step one (and sometimes even if it doesn't), I begin to build around that melody, and flesh it out.  It takes me a long time to write music, but I'm always pleased with the end composition. 

Good prog is memorable to me- that's the bottom line.  If I hear it three times and cannot recall anything, I almost always don't like it.

Captain Capricorn wrote: experiment experiment experiment ...be free in your playing & allow that freedom to open up new avenues of playing, & when you stumble upon something that resonates with your artistic vision then hone in on it & develop it to it's fullest.

Excellent way to put it.

I love the word "stumble."  My favorite things I've ever written were created by complete accident, not by "knowing what I was doing" per se.

the only advice I can give is : do something you like, that is different from what you know.

^ That feels like reading Fripp's diary  

I've only written what I would call one decent prog "melody", and as Epignosis was talking about before, it was one of those things I just stumbled upon.

Funnily enough, I was learning to play Marillion - FUGAZI on piano from ear when I just had some other random melody coming out... I've worked on it since, and its the only decent thing I've ever really written I guess.

But keep trying

I don't think you set out to write a prog song. It's just what boundaries you place around yourself and how far you're willing to push the envelope. The further you push the more likely you'll fall on your face. I've written close to a hundred songs, some pretty progressive, some very simple.

Write a bass riff on 7/8 on a major scale which contains a modulation to its relative minor scale and then make a cannon with some variations of the bass riff on guitar and vibraphone.. oh and name the piece His Last Voyage

There are a few pieces of advice I'd give, as a writer of progressive rock/metal myself.

Have what you want to achieve in mind before you write. Some people may say that art just spews out of their heads uncontrollably, but it still requires a fair bit of planning.

Write everything down, even the stuff you don't use so you can use it in another song.

The way I write music is like a jigsaw. I have a number of riffs or chord sequences all partying around in my skull and I pick a few that fit together (if I'm brutally honest, you only really need about 3 bits, 5 or 6 max, unless you want to write a 40 minutes song). Then I create little sequences that link them all together, whether it be production reliant or actual notated sequences. It's basically just a bricks and mortar process, but then again I have mild OCD so I don't know

On the scales thing, it depends whether your stuff is a major or minor, and the chords you are using. It also depends on the atmosphere you want to achieve with the solo. Is it happy? Sad? Funny? Weird? Angry? Different scales trigger different emotions in a song. When you tell me what mood you want achieve, then I may be able to help. Style of prog would also help (RIO, Prog Metal, Jazz Rock...)

I would also strongly recommend the advice Micah (p0mt3) gave you also.