I've been writing for 30 years but don't have much to show for it because of this very reason. However I think I may have had a breakthrough! It's too early to tell as I have only started the process, but so far I am a bit astounded at the results. Again, it's early to tell but I think there is real potential with this.
The concept is that first of all, like any thing else difficult, you have to practice. You are practicing creative writing, digging down for imagery and emotions. It is good to practice this in an unstructured way, no pressure about quality or rhyme, or get the meter right, no worries of melody; that will all come later. This method also suggests doing this on a timer to practice tapping the creativity quicker; I'm not sure I buy this part, but I'm following the instructions.
I'm sure you want to know what I am talking about, so I will let the proverbial cat out of the bag. It's called object writing, as i said, i have only started, but i am very surprised at the results so far Buy one of Pat Pattisons books. I have both but I think they are very similar so either will work. Or if you want his website is fairly extensive and there is a website called objectwriting.com. Find the very first post on this site, it explains how to do it.
One other thing. I think it is important to finish your songs, even if they stink. It's practice and it gets the bad stuff out of the way.
That's my 2 cents.