Solid advice from @LinusLyre. On that note, I often go for walks around my local suburb, and often end up spending some of the time singing (quietly, because otherwise people in the houses think I'm nuts! 
). I often will sing when driving to work or whatever, as well.
Also, I have a tape recording app on my phone. If I'm at home with my keyboard and I'm playing around and come up with a passage or melody I like, I'll record it. Sometimes on walks or whatever, I'll sing something or think of something and then say it into my phone. Sometimes these things come in useful later when doing more structured composing, and you'd otherwise lose them if you don't note take or record. Most smartphones these days have this kind of capability, and most can record straight to MP3, so you never need to worry about disc space. Really easy to use, and using it as essentially a dictapohone, sound quality doesn't matter too much,
While I think having structured composing time and actually limiting yourself time-wise is important as it forces you to get creative, I also think, as Patrick says, that having time that you dedicate to creatively UNFOCUS is also needed. Most of my private thinking time is on walks, and it allows your brain to stretch out of the box a little, and gather in things that you can then focus on in your more intensive composing time. Both elements are useful for a productive creative cycle.
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