Sometimes I'll go months without returning to it, but when I do get writer's block, I'm always surprised at how something I have written months, or even years ago can still mean something to me and get the creativity flowing again.  Even if you can't recall what you meant by a lyric or the meaning has changed for you, it is important to think of these little snippets as kindling to get the real creative fire going.  The muse is often an elusive little bitch and creativity tends to come in bunches; one day you might be feeling especially imaginative and write ten lines that you like for a song, but only end up using one.  Do yourself a favor next time and save the other nine because there is a good chance you will want to return to them.

Another weapon to use before your creative wellspring has become a Sahara is the voice recorder.  I use mine to take down quick guitar riffs/chord progressions on my acoustic or to sing simple melodies that I'll want to remember later.  Sometimes I'll even go so far as to recite aloud the chords progression or idea I'm using so when I return to it, I can easily figure out exactly what I was playing (this is especially helpful if the guitar was in a different tuning or strange chords were used).  While most of us record on computers now, you are going to need something that is readily available all the time, and your home studio can often take a little time to setup and get going - by then the idea has often left you and is gone for good.  Remember: it is much better to get something down that you don't use than to let ideas drift into the netherworld, never giving them a chance to be born and grow.  The gift of song is so fickle that it is important to be able to document your inspired ideas (no matter how ridiculous they might be) as soon as they come to you. 

In the fervor of the moment, it is often difficult to tell what riff or what lyrics are worth a damn.  Use this outlet as a way to create with no filter; think of all of these things you document as expendable so that you can feel free to experiment without consequence.  While much of what you get down may be useless, you may be surprised what gems you can unearth from your subconscious just by agreeing to be dutiful and wait at the alter of the muse when she calls.