It's easy to get writer's block when you're a songwriter. It's very frustrating when you get into those dry spells where you can't even pick up your guitar without getting frustrated. It's just like a novelist who stares at a blank page or computer screen waiting for something to happen. There are ways to deal with it and ways to fight your way out of it.
Inspiration can come from many places. It can strike you in the strangest of places, from the museum to the bathroom and everywhere in between. Forcing it when you're having writer's block is like torturing yourself. You need stimulus, whether it be a real life experience like a concert or from something you saw on cable. Great ways to come up with great song ideas is to read the newspaper, watch TV, surf the internet, listen to the radio and most importantly... get out and live life and inspiration will come to you.
Sometimes you can just go to the local bar and start talking to people and buying them drinks and they will tell you stories that will open up many possibilities for you to write about. Waiters and waitresses in restaurants have great stories and jokes they've come across that you can use to get the creative juices flowing. Simply sitting around the kitchen table with a bunch of friends and some drinks will spawn a great idea you can use.
Being inspired has no set formula, you just have to get out and live life if you want to write about life. You can't write songs about real issues and emotions if you've never gone out and experienced them yourself. For instance, you can't write a great song about heartbreak unless you've had your heart broken yourself and know what that feels like to you. Use that feeling and put it into your song and the passion will come through and it will explode out of you onto the paper and when you play it back, you'll be able to feel the difference in your writing.
When I write lyrics, they come from deep inside me and have a lot of passion and power that is evident in the finished product. Let your dark side come out once in a while and get people's attention with your music and they will come back for more. You will like your music that much better because it's a part of you that you are showing the world. The structure of a song is only there to reign it in once you've poured it out onto the page and molded it into what you want it to be. This is the way that great songs are born, and this is the way that inspiration will bring out the great songs that lie dormant inside you.