Hey man. Writing lyrics is a funny nut to crack. Everybody's suggestions here are great and you should try them all. You never can tell what's going to work and - for me at least - it's a little different for each song. I've definitely done the "mumbling with authority" thing!
I'll say straight out that I'm not an expert, I only know what has worked for me. I've written songs that I thought were pretty good lyrically and definitely a whole bunch that were pretty bad, even embarrassing. So you know, take this all worth a grain of salt.
Mostly I just try to get in a frame of mind where things flow a bit. I have a couple different tricks I use to get something I like. I try to build a big old woodshed of stuff. Brainstorming basically. It's good to have a backlog of material to choose from or to spur more ideas.
One of my favorite methods of brainstorming is to get relaxed (using ones method of choice), put on headphones and just listen to other peoples music - people/bands whose lyrics I really like. While I'm listening I'll get little ideas that maybe rhyme with what I'm listening to, or maybe follow the same rhythm and I write them down. Sometimes one song will really get the juices flowing and I get a couple pages of stuff out of that one song. Do this a lot with a lot of different songs and build up that backlog. Then I try to forget about all of it for long enough that I don't quite remember what songs I was listening to, and I'm not still 'in the moment'. I almost want to feel like I'm reading it for the first time when I go back so that I can be distant enough to recognize the shitty parts. The most important part is when I go back and start mining that stuff I literally throw out %90 of it and just work with the %10 that makes me think "holy shit did I even write that? That's pretty damn good" and work from there.
On totally different tip, I'll take whatever song I'm working on and just make noises wherever I can. The idea being to just figure out what spots I'm actually capable of vocalizing with. This is like what Jonathan was saying above. I play guitar and sing in my band, so figuring out where I can sing is a big part of deciding what to say. A lot of times in that process certain words or phrases pop out that just sound good. Then it's a matter of building around those spontaneous phrases.
Here's another one. Carry a little notebook with you everywhere you go. It's crazy how all of a sudden shit starts popping into your head in line at the 7-11. Plus people say crazy shit all the time, and I write that down for later. In fact, I would say the best thing you can do is just write shit down constantly hahaha. You're friends will think you're a freak, but fuckem. Generate so much material that you have no choice but to jettison most of it - the gems will shine through.
- on one last thing. It can help if you have a buddy who's taste you respect and who you can take criticism from without getting pissed to bounce ideas off of - or even collaborate with.
Good luck man! Figuring this stuff out is fun and after a while you learn what works for you and things start to click a little bit.
