I find melodies are suggesting themselves to me all the time. The lyrics are another matter altogether. I'm better at reading poetry than writing it.

I think the most important thing is to 'go with the flow'. Your creativity is likely to work best if you can record, or write down, whatever comes to you, as it comes to you. Don't ignore good lyics that pop into your head just because you're supposed to be writing the rhythm guitar part. If a tune comes to you while you're writng the lyric, [u]get it down[/u], even if you have to hum it into a tape recorder!