One common misconception about songwriting is that you generate the main ideas in one inspired session. Lyrics, music, and arrangement come flooding into your mind and your job is to transcribe it as quickly as possible.
While on lucky occasions I have stumbled on fully formed ideas like this, usually the process is much more like building a sonic Frankenstein out of odd parts you have collected. While putting all these pieces together may happen in a single inspired session, the generation of a bank of ideas occurs over a longer period of time.
I think the focus on banking ideas is often overlooked. So I say record everything. Every time you play your instrument and stumble across a passage you think is interesting, record it. Every time you hear or think of a phrase or rhyme worth remembering, write it down. Once you get in the habit of not letting any idea big or small slip by you will be documenting quite often. In no time you will have a huge archive of ideas to sort through.
The time to critique the idea’s value is not when you are documenting it. Come back to these saved ideas days, weeks, or months later. You’ll be amazed at the fresh perspective you have on something when you have forgotten about it and are listening to you would listen to a strangers idea.