Personally I've tried sitting and writing a set of lyrics and then going through and adding basic chords and building the song from there and it works sometimes. Nowadays I keep a lyric dump as per other suggestions - just type into my mobile as I'm walking about from either thinking or being sparked off by a song I'm listening to and then transfer into a notepad. I would however rate my lyric writing as poor.
A mate of mine who I write with is very gifted with lyrics and I've spent a while trying to work out quite why they work so well and it's because the guy hears and creates vocal melody lines that I wouldn't even think of which sparks off the interesting lyrics.
My personal opinion is spend time working on coming up with interesting vocal melody lines just singing "dah dah" or whatever over the top then fit lyrics in from there. Lyrics can be changed / rewritten after all but getting a good vocal melody is the true key.