Solfège works pretty well for helping train your ear to intervals.
Interval recognition is one of the most valuable tools to truly free expression in composing music. I have systematically learned all the main modes and scales on guitar which I have used in a quite boring, linear way for many years but it is only recently that I have started to play more intuitively since I began playing slide guitar where the frets that made up my normal roadmap are suddenly irrelevent and I am dependent on reacting from note to note depending on the tension or release sought in the emerging melody. I believe that should be the modus operandi of any musician regardless of their chosen instrument, medium or level of musical schooling or knowledge of theory. When I was younger and even now, I found the best way to memorise intervals is to associate them with familiar songs or melodies. For example a major second was the first 2 notes of rage against the machines ‘bombtrack’ or for a fifth i’d think of ‘don’t tread on me’ by metallica. For more complex intervals there are chordal references like the opening 4 notes to ‘oh Mr sandman, send me dream’ which is a major seventh chord. So building a vocabulary of familiar musical phrases and understanding the intervals that make them up is probably more useful for focussed composition than learning reams of theory that doesn’t necessarily equate to emotive music making
When I was in school, my professor actually had us use this exercise to help learn intervals.
Consider that this is the line between songwriter and producer and why there was two different words for them.
Practice Practice Practice.
I got given a lofi portable recorder recently and it’s just the thing for VERY quickly catching ideas for guitar or voice. No need to fire up a whole system (PC, mic, preamp, compressor, DAW, etc). This thing just sits there ready to go.
If guitar and voice ain’t your thing, then get some other quick way that works for you. Speed is the key. Maybe a template song with just a basic melodic synth or keyboard to capture the idea. Every way you look at it though the more practiced and confident you are on any instrument (or voice) and the more music just flows through you constantly, then the easier it is to hang onto those golden ideas. And if you can’t remember it because it’s too complex it may not be worth remembering?
Last night i heard a song in a dream with a female singing. I woke up and thought it would be easy to remember in the morning…yeah, right!