Take a good look at your bellybutton now and stop looking at it thereafter - what I mean is, beware of the danger of introspection; something that is important to you may not be of interest of others. (Some people advise against the use of first person - "I"; I would modify that, to allow use of fist person only if that person is not supposed to be you).
Instead, walk in other people's shoes. Put yourself in their place and judge that way how interesting your song might be.
Be alert and perceptive (I don't know if one can train themselves to this, but if you cannot do it, you're stuffed); at least try.
Choose subjects carefully; has it been done before, and if so do you think you can address the same subject better? It's best if your subjects are original (examples: Shame at inability to provide for one's own - or a child's impressions from a visit to a funfair).
Sometimes you can address a well-worn subject from an unusual perspective that gives new insights (examples: Lullaby to an unborn child - or seeing nature as the fingerprint of a creator).
In seeking originality of themes, don't forget that you are a child of your time. There are lots of new woes and joys to write about (examples: The 9-to-5-ness of most people's work experience - or the advent of child soldiers or suicide bombers).
For all that you need bags of lateral thinking (to find the unusual themes or angles). Unless you are into chemically induced widening of perception, my guess is you will find that you write best when you are freshly woken up, rested but with your mind not yet too firmly anchored on reality. I don't only write crap when I am tired, but I write only crap when I'm tired.
Here's one guideline that I use: Write the songs that you wish someone else had written; the ones you would go out and buy someone else's album for. And then be surprised and thankful that they chose to make their appearance through you.