I am usually in tesco, or in the car when some idea hits me. the triggers for the ideas are usually hearing an inspirational bit of music, seeing something cool or reading something cool. I find watching live musicians particularly inspiring.
Usually the whole idea - melody and lyric hits me at the same time. then it is just a case of working it up into a song. the idea usually (for me at least) contains some element of what i want to say.
Eg. i had the inspiration, 'you're a long time dead, so you'd better start living' once that idea was with me, it contained the thematic idea for the song, and i just wrote lyrics on advice on how not to waste your time while you're alive, get it? you can hear it here
http://www.youtube.com/watch?v=RT6VOCtYfM4
the style inspiration came from listening to Madeleine Peyroux, John Martyn and a study I was doing on gypsy jazz progressions, the lyrical one was from seeing miserable people on the metro, people working their lives away and forgetting about the fun in life
then there is structure. basically most songs will have 3 verses 3 choruses and a middle 8. I've never written a song like this in my life but it is a good starting point to aim at in that it makes you look for a structure to underlie the song.
I think also it is good advice to say that don't be afraid to write silly, childish, rubbish, nonsense, cheesy, corny, clichéd 3 chord tricks. Probably your favourite songs in the world contain one or more of these elements.
on the whole i think you can't force it, try to write art for arts sake not for any contrived or faux reason. something that helps me often is to consider the audience/listener and to think about what i want to say to that person, or people.
lastly, people who have trouble writing lyrics are usually not confident about singing. so, sing more covers and more songs by other people, you will find that not only your voice improves but your toolbox of lyrical tricks gets bigger and smarter.