Identify your audience. What works for the gospel crowd may not work for the indie-Christian crowd, and vice versa. Think about the age that listens to your genre and how best to reach them through your song.
Go toward an "explicit" or "implicit" direction with your song. Decide if you want Christianity to be the focal point of your lyric or whether you just want to introduce it in a more subtle way. This decision will affect all of your lyrics as well as how you eventually play the songs to a crowd.
Use creative metaphor or poetic phrases to illuminate Christian life. Lots of Christian musicians do well by exploring the faith in a way that hasn't been done before. Think about a novel approach to an old, but dynamic, faith that you can pen yourself.
Borrow from past great Christian song writers. Be careful though, as using the lyrics of already written songs isn't just un-Christian, it's a recipe for trouble. What you can do is evaluate those songs for the overall "method" or "technique" of the old masters to get a general idea of how you want to approach your song.
Keep thinking about how your song reflects your own personal theology. As a writer, you're going to be represented by your songs, and your lyrics are the tools you use to tell others about God. Make sure you are on the right path, as the saying goes, "to thine own self be true" and don't let your song writing be clouded by the intrusion of others.