I often get little ideas like a riff or lyrical phrase in my head that I might mess with a few hours or so. If I don't have a guitar around, I find something to write down some words and chords as quick as I can, maybe put something on my phone voice recorder, and then come back to it later with a guitar when I have time. Usually, I have to spend some time with it initially before it really catches hold, though. Very rarely do I find ideas to use when I go back through them if they don't at least have some complete hook, verse, or chorus.
Out of laziness, sometimes I don't write or record anything, and it's gone later in the day. However, I've also found that if something doesn't come back to me easily a few days later, then maybe it wasn't that great to begin with. Songs that I finish are rarely ones I had to record anything before I would consider it done enough.
I will say that the songs I write tend not to be very complex, and I do have a tendency to repeat myself. It feels a little self-defeating at times, but the reality is that so few people are going to hear my songs, anyway, much less care if they notice something sounds like anything I wrote previously.
A professor in a songwriting class I took years ago would always say to start with lyrics, then build music to it. Sometimes that works, but starting from a riff, and imagining sounds of words that fit has worked just as well for me. I do find that if I can get a hook with some lyrical content together roughly in the same time frame then it tends to go further than either way alone.
Over the years I have tried to be more systematic and consistent. But I have so many hobbies I cycle through that I kind of have to just go with it when inspiration hits, and hope that I can get enough done with when I initially work on it to either stay motivated to finish it, or feel inspired enough to finish it if and when I go back to it later. Some of what I would consider my best songs were ones I wrote in a day, as well as ones I farted around with for several years.
One thing I should do more of is treating songs as their own entity, stay with them until they are absolutely polished and done before performing or recording them. Same for producing - I rarely go back and re-record or produce a song I've already tracked and mixed. I should really take the next step, get into a studio, hire a real singer, and make some decent demos. At that point the return of investment is a gamble, though, and a risk I'm generally not inclined to take. Music is my hobby, but not my only one.
