Hmm this is a hard question. I have five, but these aren't necessarily in order of importance.
1) Listen to lots and lots of music. Maybe genres that make you uncomfortable or that you just plain don't like. All sorts of weird stuff. Try and absorb as much as possible because if you listen to one artist or one genre forever, your music will stagnate and become boring.
2) LEARN THE RULES. This isn't in regards to music theory, but more song structure. Study pop music, study classic rock, study hip-hop - write it over and over and over again to the best of your ability. And this doesn't just go for people who are trying to make commercial recordings, or trying to break the pop charts. Even if you want to be the most insane musical maverick this century, you have to know what makes genre music sound the way it does, and what makes it work, before you can successfully destroy these definitions and make whatever avant garde discofusion deathdrone you want to make. The only reason totally mad music works is because the artist knows the rules, and is deliberately turning them on their head.
3) Melody first, words after. This has always been my general rule and for people who want to write inventive, catchy and surprising melodies, I cannot stress it enough. Unless I am writing folk or something that is very wordy where the main focus is the lyrics, this is my method. There's nothing wrong with coming up with a line or two as a starting point and constructing a song around that, but generally coming up with a tune without the restriction of the words is your best bet for writing inventive melodies because you write independently of preconceived structures.
4) Write every day, especially if you are just starting out. Even if you throw away everything you write and think it's utter crap, keep writing. Eventually you'll hit your own flow. I keep (ie use in live sets, finish basic song, record or commit to memory) maybe 20% of everything I write, but song writing is an exercise in logic as well as creativity and it helps to use those muscles just like any other past time. If you can't already, eventually you'll be able to sit down and write how you want, and if you're really, really lucky in your own, distinctive style.
5) Finally, there's nothing wrong with wearing your influences on your sleeve. David Bowie once called himself a 'tasteful thief'. The real knack to art in this day and age is knowing what to steal and how to incorporate all these different elements in to something cohesive. That's what's fun. When I write I don't even worry about people saying to me, "Well, you've clearly gotten that from here." I'll just say, "So what?"
