There is a great book by Robin Frederick called "Secerets to Hit Song Writing" cheesey name but really packed full of good info and tips. I keep a little book that I write stuff donw in. I'll hear a phrase or comment from someone and if it strikes me as a lyric I write it down. Even if you don't have a song in mind, just write down everything and you can go back to it later when you need a verse. Same for riffs and chord progessions you come up with. You can always take a verse, take each word and then list all the words that are similar and then all the words that are opposite. YOu can usually come up with some interesting things that way. Sometimes you have to know when to walk away from it for a bit too. You'll come back fresher if you have had a bit of a break instead of trying to force something when you are frustrated.
Just my thoughts.
MJ