I only write two or three great songs a year. The rest of the time, I write junk — horrible lyrics, horrible concepts, bad melodies. Seems like a waste of time, but it’s all part of a process. Here’s how I approach my songwriting:

1. Keep writing. According to one of my journalism professors in college, Ernest Hemingway once said, “Your first millions words are shit.” You aren’t going to write great things right off the bat. Songwriting employs skills that must be practiced and honed. You’re practicing rhyming, scansion, meter — all the things you need. So when that great song idea comes, you’ll have the skills to keep up with your muse.
2. Write Every Day. Even when you don’t feel like writing. Even if you think (or know) you’re not going to write anything worthwhile. It’s like going to the gym.
3. Record Ideas Immediately. I only actually get down about half of the ideas I come up with. But if you don’t make the effort to write them down, you risk losing your ideas to the ether.
4. Don’t Throw Anything Away. Keep everything. Every scrap. One day, you might look through a collection of your old, crappy ideas, and one of them engenders a great idea. All creative effort has value, even if only to its author, so keep it all.

Here are a few links to more creative tips.
Lifehack: How to Boost Your Creative Output – I love lifehack.org. Lots of articles to help you become more organized, efficient, creative, etc.

Get Creative: Part 1 and Get Creative: Part 2 – Although I can’t imagine how to write songs in a hot tub — not while keeping the guitar dry.

10 Mental Blocks – I like these kinds of articles. Rather than tell me what to do, tell me how I’m screwing up.